Mr Rubio only won election to the US Senate in the 2010 mid-term elections
Florida senator and conservative favourite Marco Rubio has endorsed Mitt Romney in the race for the US Republican presidential nomination.
Mr Rubio, speaking on Fox News, said Mr Romney would be an impressive president and an improvement over the “disastrous” Barack Obama.
He would not be drawn on frequent speculation that he could be named as Mr Romney’s vice-presidential pick.
On Thursday former President George H W Bush will also endorse Mr Romney.
Marco Rubio is popular among very conservative and Tea Party voters who have remained sceptical of Mr Romney throughout his candidacy.
In addition, his Hispanic roots are thought by analysts to offer a key opportunity to win the support of crucial Latino voters.
Speaking to Fox News host Sean Hannity, Mr Rubio said: “I am going to endorse Mitt Romney.
“Not only is he going to be the Republican nominee, he offers a stark contrast to this president’s record.”
He brushed off concerns that Mr Romney had struggled to dominate the lengthy primary process, and said he had no doubt the former governor of moderate Massachusetts would campaign and govern as an authentic conservative.
“This primary has been excellent because it has meant people take position on issues that we can hold them accountable to,” he added.
Mr Rubio’s endorsement is the latest high-profile Republican figure to fall in line behind Mr Romney, who is widely seen as the inevitable eventual nominee.
In a statement, Mr Romney said his latest backer was a living example of the American Dream.
“From humble origins, he has risen to become one of the brightest lights in our political party.
- Mitt Romney
- Rick Santorum
- Newt Gingrich
- Ron Paul
A candidate needs 1,144 delegates to win
“He has been a leading voice in the US Senate and the country for the cause of restoring American greatness.”
Former Pennsylvania Senator Rick Santorum remains in second place in the primary race, but is far behind Mr Romney with roughly half of the number delegates won.
Mr Santorum appeals to conservatives, in part because of his message of social conservatism, and has won a string of states in highly conservative and religious areas of the US.
But Mr Romney has now picked up the endorsement of two key party power-brokers in 24 hours – the elder statesman George H W Bush and Mr Rubio.
Mr Bush and Mr Romney will appear together on Thursday in Houston, Texas, Romney spokeswoman Gail Gitcho said.
Another heavy-hitting conservative figure from Florida, former Governor Jeb Bush, gave Mr Romney his support a week ago.
Meanwhile, Mr Romney’s other mainstream rival Newt Gingrich is reported to be struggling to keep his campaign.
He has dramatically reduced his staff and cut his campaign events, reportedly to focus on winning the nomination at the Republican convention in Tampa in August.