[cnn-photo-caption image= http://i2.cdn.turner.com/cnn/2010/images/04/16/art.romneyrubio5.gi.jpg caption ="Mitt Romney (right) announced Friday that he is endorsing Marco Rubio for Senate in Florida."](CNN) - Former Massachusetts Gov. Mitt Romney is no longer staying on the sidelines in Florida's closely-watched Senate Republican primary.
A Romney adviser confirmed to CNN Friday that he will endorse former House Speaker Marco Rubio over Gov. Charlie Crist, a decision made considerably easier following Crist's veto of an education bill popular among Republican legislators and some conservatives.
The two Republicans will campaign together in the vote-rich Tampa area on Monday and Romney's Free and Strong America PAC will contribute the maximum $5,000 to Rubio's primary campaign.
"Over the past many months, Governor Romney has spoken with both Governor Crist and Marco Rubio," a Romney adviser told CNN. "Both of them asked for his endorsement. While Mitt Romney respects Governor Crist, he believes Marc Rubio's record of conservative, principled and idea-driven leadership is what Florida needs now."
Romney is the latest in a long line of national GOP figures to back Rubio, whose campaign has galvanized conservatives and Tea Party activists around the country frustrated with the Obama administration and government spending.
With a few exceptions, the roster of potential 2012 presidential candidates, Romney included, have largely avoided taking sides in contested Republican primaries.
But as state and national Republicans hammer Crist for his veto, and with the governor now rumored to be weighing an independent bid, the political risk of picking a horse in the GOP primary lessens considerably. According to a Quinnipiac poll released Thursday, Crist trails Rubio by 23 points in the primary.
Romney's endorsement of Rubio also represents a bit of political payback. As the 2008 Florida presidential primary boiled down to a battle between Romney and Sen. John McCain, Crist threw his support at the last minute to McCain.
Romney is the second potential 2012 candidate to wade into the primary; former Arkansas Gov. Mike Huckabee endorsed Rubio last year. Former Alaska Gov. Sarah Palin has also expressed support for Rubio but has not formally backed his bid.
UPDATE 4:30 p.m.: The Democratic National Committee couldn't resist the chance to take a dig at Romney after the endorsement, accusing him of flip-flopping on the race.
" We all understand that Mitt Romney's endorsement of Rubio at this stage certainly isn't much of a game-changer, but it certainly highlights his propensity to be, what can charitably be called, a mind-changer. Charlie Crist should take heart – you never know when Romney could flip flop again," DNC spokesman Hari Sevugan said Friday in a statement.