The former Massachusetts governor will announce the endorsement Saturday, according to the Associated Press.
Romney is the latest in a long line of 2008 presidential candidates to endorse the former state House speaker, a favorite of the conservative movement and a darling of the Tea Party. Former New York Mayor Rudy Giuliani and former Arkansas Gov. Mike Huckabee have also endorsed Rubio.
The AP quotes a Romney adviser as saying both Gov. Charlie Crist (R-Fla.) and Rubio sought Romney's endorsement and that Romney believes "Rubio has a stronger conservative record and is an idea-driven leader." Romney's PAC is giving Rubio $5,000, and the two men will campaign together on Monday.
Rubio raking up former presidential endorsements is fun for political watchers to follow. During the 2008 presidential race, several candidates sought Crist's endorsement before the Florida primary. He picked Sen. John McCainJohn McCainThe trouble with Rex Tillerson Senate: Act now to save Ukraine A Cabinet position for Petraeus; disciplinary actions for Broadwell after affair MORE (R-Ariz.), which helped McCain win the state primary, which in turn aided McCain in securing the GOP nomination.
Rubio has also sought former vice presidential candidate Sarah Palin's support, but she hasn't formally announced yet.
Romney's action will no doubt fuel speculation Crist may run as an independent. Crist vetoed a GOP-backed education bill on Thursday, which political watchers saw as a sign he's considering going independent. And a Quinnipiac poll found in a hypothetical three-way
race, with Crist running as an independent, the Florida governor would win
with 32 percent of the vote over Rubio's 30 percent.
Crist has until April 30 to decide. That's the deadline for filing for the Florida ballot. The primary is Aug. 24.