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It’s been a volatile primary season as three men, vying to become the Republican nominee for an Indiana Senate seat, fight to prove which one is more conservative and loyal to President Trump.

The primary race features two U.S. congressmen -- Reps. Todd Rokita and Luke Messer -- and businessman Mike Braun, who Politico has dubbed Indiana’s “mystery man.” All three candidates have promised voters they can defeat the incumbent Democrat.

Incumbent Sen. Joe Donnelly, a Democrat, has held the office since 2013. While Donnelly earned Trump’s ire when he didn’t support Republicans’ tax reform plan, he has bucked his own party’s traditional viewpoints for that of more conservative ones. He’s anti-abortion and was one of only three Democrats who voted to confirm Supreme Court Justice Neil Gorsuch, the president’s pick.

“I’ve voted with the president 62 percent of the time,” Donnelly has told Fox News. “I’ve voted for a whole bunch of his nominees … we’ve worked together on the opioid issue. We’ve worked together on veterans issues.”

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Todd Rokita, Luke Messer and Mike Braun have been locked in a volatile and contentious primary for the Republican nomination for Senate in Indiana. (AP Photo/Darron Cummings)

The race between Donnelly and whoever wins the Republican primary is considered a toss-up.

Read on for a look at the Republican candidates ahead of today's primary.

Todd Rokita

Indiana Senate GOP Candidate

Rep. Todd Rokita is passionate about pushing immigration reform, including banning sanctuary cities and establishing English as the national language. (AP Photo/Darron Cummings)

Rokita has two advantages, he says: defending Trump and the ability to beat the incumbent Democrat in the fall.

“The race is coming down to who Hoosiers can trust to beat Joe Donnelly and stand with our president,” Rokita told Fox News.

And Rokita, 48, has a reputation in Indiana of being tough enough to avoid backing down from a fight.

“He will be a bulldog. He will be in Donnelly’s face,” former state Rep. Mike Murphy told the Indianapolis Star.

Rokita was first elected to the U.S. Congress in 2010, where he represents Indiana’s fourth district, west of Indianapolis. He serves as the vice chairman of the powerful House Budget Committee as well as sits on the Education & Workforce and Transportation & Infrastructure committees.

More on the Indiana Senate race:

Prior to Congress, he served as Indiana’s secretary of state where he pushed for the state’s voter identification law that got approval from the U.S. Supreme Court in 2008.

As a candidate, Rokita has addressed anti-abortion issues, gun rights and term limits. He’s also called for stricter immigration laws -- including “slapping handcuffs on politicians” who aid in sanctuary city policies -- and establishing English as the national language in the U.S.

“It’s those issues that will really make the difference and where you need courageous conservatives,” Rokita said.

He's been an ardent defender of Trump and has insinuated that he will help “stop the witch hunt” against the president in a recent television spot that includes images of special counsel Robert Mueller, House Minority Leader Nancy Pelosi and fired FBI Director James Comey. He called Messer -- in the ad and in interviews -- a “Never Trumper.”

But, as Rokita’s opponents are quick to point out, he wasn’t always aboard the Trump train. Rokita criticized Trump as “vulgar, if not profane” during the presidential primary and propped up Florida Sen. Marco Rubio as his candidate of choice.

As the Indianapolis Star reported, Rokita has been the first to attack his opponents in multiple debates.

With only a few days to go until the primary, Rokita said he has embarked on a 50-city tour of the state and maintains he’s more well-versed with Indiana than his opponents.

“The reason I’m doing this, it’s not for my health,” Rokita joked. “You have to see people face-to-face, and I’m the one who's best at it. It will take me to beat Joe Donnelly.”

Rokita graduated from Wabash College in Indiana before he earned a law degree from the Indiana University School of Law. He and his wife have two children.

Mike Braun

Braun

A businessman, Mike Braun likens himself to President Trump as a political outsider. (AP Photo/Darron Cummings)

Mike Braun wants to stand out. A self-described outsider in the race, Braun knows he’s different from his opponents. He’s not a congressman. He didn’t attend law school. And he’s a little older.

During the debates, while Messer and Rokita donned jackets and ties, Braun just wore a dress shirt. His outsider-status likens him to Trump, Braun said.

“President Trump, he was a disrupter, outsider and businessman, and look what’s occured,” Braun, 64, told Fox News, praising the president for scaling back on regulations. “I’ve lived it. I’ve signed thousands of paychecks just like the president did [as a businessman]. He was an inspiration for me to run for senator, and I hope for others as well. There needs to be more of us to step outside the comfort zone.”

But Braun does have some political experience. He served as a state representative for three years in Indiana.

Braun’s campaign website bills him as a “conservative businessman,” but -- as his opponents have pointed out -- he voted with Democrats for more than a decade in state elections. His campaign spokesman has maintained that he only voted for Democrats to disrupt the outcomes of those races.

Still, Braun contends his business acumen better positions him to work with the president on infrastructure, health care and economy issues.

“I won’t need to be briefed on important issues when I get [to Washington]. I’ve had to live through them,” Braun added. “I think if we had eight or 12 of us in the Senate who came from my business background, you’d see a lot of different things in government. The Senate is where the issues are, and most people there are career politicians.”

If elected, Braun said he will not collect a pension and would advocate that lawmakers should be on a 401(k) plan instead. Similarly, he said lawmakers should not get special health care plans.

“I think that for many people, that puts salt in the wound of poor performance when congressmen and senators have those kinds of advantages and perks,” he said.

"Trump was an inspiration for me to run for senator, and I hope for others as well."

— Mike Braun

Aside from business issues, Braun said he would focus on gun rights, border security and strengthening the military.

Braun attended Wabash College in Indiana and graduated from Harvard Business School with an MBA. He then moved back home and started Crystal Farms, Inc., which would grow into one of the largest turkey operations in the Midwest, according to his campaign website. Eventually he would become the owner of Meyer Logistics and establish Meyer Distributing with employees across the country.

He and his wife have four children.

As for the days leading up to the primary, Braun said he isn’t clamoring to make any changes.

“It’s like a business plan. If you’re scrambling to make adjustments a few days before you roll out something, whether it’s a sales strategy or campaign, you probably didn’t do a good job leading up to it,” he said. “I executed the game plan. But I’m not going to relax until the votes are in Tuesday evening.”

Luke Messer

Luke Messer

Rep. Luke Messer, like his opponents, has highlighted his support for President Trump. He led the effort by House Republicans to formally nominate the president for a Nobel Peace Prize. (Michelle Pemberton/The Indianapolis Star via AP)

With only a few days left until the primary, Rep. Luke Messer is proud of the grassroots effort of his campaign. That, plus his efforts to “keep our campaign focused on supporting the Trump agenda,” give him the edge in the primary, the congressman told Fox News.

“I am who I say I am,” Messer replied when asked what gives him an advantage over his opponents. “I have the best record of supporting President Trump.”

Messer, 49, already represents Hoosiers living in the 6th congressional district, in the southeastern part of the state that encompasses some Cincinnati and Indianapolis suburbs. Elected in 2012, he sits on the Education & Workforce and Financial Services committees.

Like his opponents, Messer believes he’s the one who sticks with Trump the most. He spearheaded an effort by 18 House Republicans to formally nominate Trump for a Nobel Peace Prize because of his efforts in the Koreas.

“I think President Trump showing peace through strength is working, and the only reason that the evil dictator in North Korea is coming to the table is because this president has stared him down,” Messer previously told Fox News. “And as peace comes to the Korean peninsula, I do believe President Trump deserves the Nobel Peace Prize.”

Messer praised Trump for an uptick in the economy and for putting “ISIS on the run.”

The main issues for Messer, according to his campaign website, are gun rights, anti-abortion policies, immigration, health care and school choice. He also calls for a border wall to be built and a “crack down” on so-called sanctuary cities.

Messer attended Wabash College in Indiana and received a law degree from Vanderbilt University. He worked for Koch Industries and was selected to serve in the state House in 2003, which became a point of contention during the campaign after it was first reported by the Indianapolis Star that Messer might have been less-than-forthcoming about his own DUIs when he was tapped to replace the seat of a state lawmaker killed by a drunk driver.

A campaign spokesman told Fox News Messer had already “acknowledged and apologized for these mistakes” that happened more than two decades ago. Messer’s campaign also blamed Rokita for the story, calling it a “last minute dirty trick from Rokita’s failing campaign.”

After working in the state legislature, he became a lobbyist. And when he ran for Congress in 2000, he lost to Mike Pence, now the vice president.

Messer credits his mother, who raised him and his brother alone, for pushing him to be successful. He and his wife have three children and authored a children’s book, called “Hoosier Heart: A story about Hoosiers and the Great State of Indiana.”

(Rokita’s camp, too, has written a book -- although it is a parody of Dr. Seuss’ “Oh the Places You’ll Go” and is used to attack Messer.)

As for his opponents, Messer has called Braun a Democrat and implied Rokita was less than honest with controversial campaign signs that, at least from a distance, appeared to show an endorsement from the president. The signs said the Rokita campaign was endorsed by the “Trump/Pence 2016 Indiana Team Leaders.” However, the names of the president and vice president are in much larger font than “2016 Indiana Team Leaders.”

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Donald Trump's re-election campaign has issued a rebuke to Todd Rokita, ordering the Republican to take down yard signs that could give a false impression that Rokita is endorsed by the president, two officials with direct knowledge of the matter told The Associated Press. (AP Photo/Michael Conroy)

Messer, himself, has been slammed as a “Washington resident,” as his wife and children live in Virginia.

“I’ve got a home in Indiana that I’ve owned for 17 years,” Messer previously told Fox News. “My kids do need a full-time father, but I’ll tell you this -- that’s not what Hoosiers are focused on.”

With only a few days left until the primary, Messer is still traveling around his state. He said he plans to visit dozens of communities in the final days of the primary.

Fox News’ Peter Doocy and The Associated Press contributed to this report.