Florida Rep. Patrick Murphy will announce his campaign Monday to seek the Senate seat expected to be left vacant by Marco Rubio's likely presidential bid. 

The second-term Democrat -- who won national exposure and the affection of many in his own party when he unseated firebrand tea party Rep. Allen West in 2012 -- confirmed his candidacy to The Associated Press, making him the first to formalize a bid for the seat. 

Murphy, 31, will run regardless of Rubio's decision, and though he could face tough competition, he focused his initial attention squarely on the first-term senator. 

"For years, Sen. Rubio has put the needs of Floridians behind his presidential ambitions," Murphy said in a statement. "We need a leader in the Senate whose eyes are firmly fixed on the people of Florida by working together to get things done." 

Murphy immediately becomes a formidable candidate whomever his eventual opponent. 

He defeated West as a 29-year-old political novice, in a district that tilts slightly Republican. Two years later, in a re-election bid many anticipated would be close, he easily turned away his opponent. 

He has been a prolific fundraiser who has won many voters with his moderation, but he has never faced much in the way of a primary. His aisle-crossing, likely to be seen as an asset in a general election, could become a target for a Democratic foe in the primary. He was previously a Republican who donated to the 2008 campaign of Mitt Romney, among others. And he has parted ways with the Democratic leadership, voting to create a House committee to investigate the Benghazi, Libya, attack that killed four Americans; voting in favor of the Keystone pipeline; and co-sponsoring the GOP-led "Keep Your Health Plan Act," which would have allowed insurers to continue offering plans that didn't meet Affordable Care Act rules. 

In a statement to be released Monday, Murphy called himself "a consensus-builder" and "an independent voice." 

"Washington is full of hyper-partisan politicians who can't or won't get anything done and Florida deserves better," he said. 

Murphy's announcement comes after two higher-profile Democrats -- former Gov. Charlie Crist and Democratic National Committee chairwoman Debbie Wasserman Schultz -- said they wouldn't run, easing his way to the nomination. Among others who have expressed interest in the race is Rep. Alan Grayson, posing the chance of a primary matchup between a controversial liberal and the much more moderate Murphy. 

Rubio has said he would not run for re-election to the Senate while pursuing the presidency, and a number of Republicans are said to be eyeing his seat, including Lt. Gov. Carlos Lopez-Cantera and Jeff Atwater, the state's chief financial officer. 

Murphy is a born-and-raised Floridian who was an accountant before working for his family's construction company. In launching his political career, he targeted West, who he said was "an embarrassment to the country." When redrawn boundaries made West's district far less favorable to the GOP, the Republican bolted for one further north and Murphy followed. The resulting contest was epic, culminating in weeks of recounts and court appearances before West finally conceded. 

It still ranks as the most expensive House race in U.S. history, and made Murphy the youngest member of the 113th Congress.