Illinois Sen. Mark Kirk has become the most recent GOP Senate incumbent to have his reelection campaign ensnarled in the politics of replacing Supreme Court Justice Antonin Scalia, breaking with Republican leadership on the issue.

Kirk on Friday became the first Republican senator to call for a vote on President Obama's Supreme Court selection, appellate Judge Merrick Garland.

"It's just man up and cast a vote," Kirk said.

The first-term senator in 2010 narrowly won the Senate seat once held by then-Democratic Sen. Barack Obama.

And he now faces a difficult general election challenge in the blue state from Democratic Rep. Tammy Duckworth, considering the Army veteran largely has the backing of Senate Democratic leadership.

Washington Democrats have primarily targeted Kirk, along with fellow GOP Sens. Ron Johnson of Wisconsin, Kelly Ayotte of New Hampshire, Rob Portman of Ohio, and Pennsylvania’s Pat Toomey in their efforts to take control of the upper chamber.

Republicans have a 54-46 majority in the Senate and must defend incumbents in 24 of 34 races.

Nathan L. Gonzales, editor and publisher of the nonpartisan Rothenberg & Gonzales Political Report, said earlier this month that the five targeted races along with GOP Sen. Marco Rubio’s open seat in Florida and Democrats having to defend Senate Minority Leader Harry Reid’s open seat in Nevada will likely determine which party controls the chamber next year.

He also said Democratic strategists think Republicans' lack of action on Obama’s Supreme Court nominee will give them a fresh issue, while Republican strategists believe the issue could energize their conservative base.

“The Senate majority is still firmly up for grabs, with a broad range of potential outcomes,” Gonzales said.

Even Iowa GOP Sen. Chuck Grassley, a 35-year incumbent and chairman of the Senate Judiciary Committee, is facing stronger re-election headwinds because of the issue.

He is backing Senate Majority Leader Mitch McConnell in saying Americans want to wait for the next president, who takes office in nine months, to nominate a replacement for the conservative-minded Scalia, who died in February.

Democrats have now gotten a popular, statewide figure -- former Iowa Lt. Gov. Patty Judge -- to run against Grassley, after basically conceding the race this fall in the conservative-leaning state.

Tom Lopach, executive director of the Democratic Senatorial Campaign Committee, said Grassley’s “unprecedented obstruction of the constitution and flat out refusal to … hold hearings on a Supreme Court nominee are proof that he’s simply spent too much time in Washington following his party at the expense of common sense.”

Meanwhile, at least one conservative group, the Judicial Crisis Network, is reportedly trying to protect vulnerable Senate Republicans by spending millions on attacks ads against Colorado Sen. Michael Bennet, considered the most vulnerable Democratic incumbent, and Democratic Sens. Joe Manchin of West Virgina, and Heidi Heitkamp of North Dakota.

Manchin and Heitkamp are considered vulnerable on the Supreme Court issue in their conservative-leaning states in their 2018 re-election bids, as reported first by Politico.

Kirk said he believes McConnell won't relent, saying, "I don't see his view changing too much."

Gonzales also said other states -- including Colorado, North Carolina, Missouri and Arizona -- could come into play as Democrats try to gain a net five Senate seats, based on top-of-the-ticket dynamics, particularly whether unpredictable GOP front-runner Donald Trump wins the party's nomination.