If Marco Rubio fulfills expectations and announces Monday that he is running for the 2016 Republican presidential nomination and not re-election to the Senate, Florida's chief financial officer and a Democratic congressman will become the leading candidates to replace him.

Republican CFO Jeff Atwater would be the immediate favorite to replace Rubio after dominant wins in two statewide campaigns. Democratic U.S. Rep. Patrick Murphy is not nearly as well-known. But he is seen as a moderate who can win crossover votes after defeating incumbent Republican Rep. Allen West in 2012 despite West's massive fundraising and national tea party support.

The race could be an opportunity for Democrats to pick up a seat as the party has a history of doing better in Florida during presidential years. President Barack Obama carried the state in 2008 and 2012, but Republicans won the governor's race and most other statewide races in 1998, 2002, 2006, 2010 and 2014. Democrats hold a 39 percent to 35 percent advantage in voter registration over Republicans.

"Florida is a different state in a presidential and a non-presidential year," said Democratic pollster David Beattie. "Regardless of who's on top of the ticket, turnout goes up. Being on the ballot on a presidential year is better for the Democrat."

Murphy, 32, whose district includes parts of Palm Beach County, didn't wait until Rubio's announcement to get in the race. He says he isn't concerned about a possible primary or potentially facing a better-known Republican.

"The issues that I support and my stances cut across the political spectrum," said Murphy, adding that he's strong on fiscal issues and supports keeping abortion legal and increasing the minimum wage. "In our last election we were humbled to have a strong victory in a district that Mitt Romney ended up winning by a few percentage points."

Democrat Sen. Bob Graham held the seat from 1987 until he retired in 2005. It was then won by Republican Mel Martinez, who left office before finishing the term. Then-Gov. Charlie Crist appointed Republican George LeMieux to finish the term. Crist then ran for the seat himself, first as a Republican and then as an independent. He lost to Rubio, who says he won't seek re-election if he gets in the presidential race even though he could — the deadline for entering the Senate race is May 6, 2016, after the presidential nomination will likely be decided.

Democratic U.S. Rep. Alan Grayson of Orlando is another possible candidate, but many see him as too liberal and too inflammatory to win statewide. He recently had a messy divorce trial where he accused his wife of bigamy before they agreed to an annulment. Some Democrats are advising him not to run.

Atwater, 57, hasn't officially entered the race and won't until after Rubio makes his announcement on Monday in Miami. However, a political committee has already been formed to support him, and he is calling supporters to let them know he is preparing to run.

Atwater could face a primary challenge from U.S. Rep. Ron DeSantis and Lt. Gov. Carlos Lopez-Cantera. One plus for DeSantis, who represents a district that includes Daytona Beach and St. Augustine, is that he would be able to transfer about $1 million from a House campaign account to a Senate campaign.

"You expect other people to be interested in a race this size," said Republican political consultant Rick Wilson, who is advising Atwater. "It's a challenge that people who haven't run statewide before underestimate."

Atwater is the only candidate considering a run to have won a statewide race on his own. He's a former bank president who served as state Senate president before being elected CFO in 2010, a race he won with 57.3 percent of the vote — a larger margin than Gov. Rick Scott, Rubio or the other two Cabinet members on the ballot that year. But he's also never faced a primary opponent in a statewide race.

While Lopez-Cantera also won statewide office, he did so as Scott's running mate and he isn't as well known to Floridians. It wouldn't help, either, that Scott's unfavorable ratings have been high since the day he took office in 2011. Scott is the only Florida governor to win office twice without receiving a majority of the votes cast.

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