A number of high-profile Republican legislators have recently backed Romney, including Sen. Marco RubioMarco Antonio RubioOvernight Defense: Tillerson, Trump deny report of rift | Tillerson says he never considered resigning | Trump expresses 'total confidence' in secretary | Rubio asks Army to kick out West Point grad Rubio asks Army to kick out West Point grad with pro-communist posts GOP establishment doubts Bannon’s primary powers MORE (R-Fla.) and House Budget Committee Chairman Paul RyanPaul RyanThe Hill Interview: Budget Chair Black sticks around for now Gun proposal picks up GOP support GOP lawmaker Tim Murphy to retire at end of term MORE (R-Wis.), the author of a budget proposal that has served as a conservative litmus test. The endorsements are a boon for Romney and a further blow for Rick Santorum, Newt Gingrich and Ron Paul, all of whom are trailing in the number of delegates and endorsements.

On Sunday, Senate Minority Leader Mitch McConnellAddison (Mitch) Mitchell McConnellGun proposal picks up GOP support Children’s health-care bill faces new obstacles Dems see Trump as potential ally on gun reform MORE (R-Ky.) said that the Republican nominating process was virtually over and that Romney was clearly the GOP's 2012 presidential nominee. McConnell did not, however, endorse Romney.

"I think he’s going to be an excellent candidate and I think the chances are overwhelming that he will be our nominee," McConnell said on CNN's "State of the Union." "It seems to me that we’re in the final phases of wrapping up this nomination and most of the members of the Senate Republican Conference are either supporting him or have the view that it’s time to turn our attention to the fall campaign and begin to make the case against the president of the United States."

In the same interview on Monday, Portman, whose name has come up on some shortlists for the No. 2 spot on the Republican presidential ticket, said he was "not interested" in running for vice president.