GOP presidential candidate Mitt Romney is staying tight-lipped about his coming running mate selection, but that hasn’t stopped intensifying speculation over the decision.

“I have nothing for you on the vice presidential question,” Romney said when asked if he had made up his mind during an interview aired Sunday on CNN’s State of the Union.

“I give you nothing on that. But I can assure that by the third day of the Republican convention we will nominate a Republican VP,” he added.

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But other political heavyweights are placing their bets on who will join the GOP presidential ticket.

South Carolina Sen. Lindsey GrahamLindsey GrahamOversight panel demands answers on Pentagon waste report Overnight Cybersecurity: Retired general picked to head DHS | Graham vows to probe Russian election interference Overnight Tech: AT&T, Time Warner CEOs defend merger before Congress | More tech execs join Trump team | Republican details path to undoing net neutrality MORE (R) placed his money on former Minnesota Gov. Tim Pawlenty (R) and Ohio Sen. Rob PortmanRob PortmanSenators to Trump: Get tough on Russia over Ukraine John Glenn dies at 95 John Glenn hospitalized MORE (R), both of whom are rumored to be on Romney’s short list.

Florida GOP Rep. Mario Diaz-Balart tossed his weight behind fellow Sunshine State lawmaker, Sen. Marco RubioMarco Rubio House passes water bill with Flint aid, drought relief What the 2016 election can tell us about 2018 midterms Fight over water bill heats up in Senate MORE (R).

"I think Marco Rubio would be a spectacular candidate for vice president. I think he would energize the country. I think he has a great story,” Diaz-Balart said on CBS’s “Face the Nation,” noting that Rubio has “more” elected experience than Romney did. 

Former Democratic Govs. Ted Strickland (Ohio) and Ed Rendell (Pa.), whose states are key battlegrounds also weighed in with their predictions.

Strickland said he expected Pawlenty to get the nod over Portman, noting Portman’s role as then-U.S. Trade Representative and Director of the Office of Management and Budget (OMB) under former President George W. Bush.

“He was the budget director at a time when we were careening towards the conditions that led to this recession," Strickland said, adding that Pawlenty, however could attract evangelical voters for Romney.

Rendell joked that he wanted lightning-rod conservative and one-time presidential hopeful Rep. Michele BachmannMichele BachmannWill Trump back women’s museum? Michele Bachmann on Trump victory: ‘God did this’ The right-wing wants a revolution, and we had better pay attention MORE (R-Minn.) on the GOP ticket.

“I’m for Michele Bachmann for vice president. I want everyone to be clear about that," he said laughing. 

Rendell also applauded Romney for saying he would choose a running mate with enough experience to step into the role of president should it become necessary.

Virginia Gov. Bob McDonnell (R), who has also been the focus of speculation, however, followed Romney’s lead and refused to share his thoughts on the decision, refusing to even confirm if he has been vetted.

McDonnell told CBS host Bob Schieffer, “as soon as I find out, I'll let you know.”