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In a memo obtained by multiple media outlets Monday, Santorum's campaign acknowledged that they were unlikely to secure enough delegates to win the GOP nomination, but suggested that the 1,144 total could also be difficult for Mitt Romney, opening up the possibility of a brokered convention.

Trump said Santorum's 2006 Senate loss — the largest margin of defeat for an incumbent senator since 1980 — disqualified him as a legitimate contender. The real estate mogul went on to argue that any benefit from the prolonged primary had already been realized, and Republicans should now focus on the general election.

"Romney's going to win, and I think it's now time for us to focus on Obama, which we're not doing," Trump said.

Trump announced last week that he would record robocalls for the former Massachusetts governor that could go out in all 50 states.

“Voters across the United States should expect a call from Donald TrumpDonald TrumpLiberals don't understand difference between news and opinion Rubio: Former campaign aides targeted by IP address in Russia Ivanka Trump will be an asset in her new role at the White House MORE praising Mitt Romney for his tough stances on China, OPEC and other foreign countries that are depriving Americans of opportunities and jobs so that they can fulfill the American Dream,” Trump spokesman Michael Cohen told ABC News.