WASHINGTON -- Marco Rubio may have said it best himself, when talking about what he could bring to the ticket as one of Mitt Romney’s potential vice-presidential running mates: "Presidential campaigns are won by the presidential nominee."

Rising Republican star though he may be, the Florida senator’s national appeal may be tepid among the Hispanic voters both parties are courting so desperately this election year. Whoever wins the presidential election will do so in part by winning over a majority of the estimated 12 million registered Latino voters. About two in three of them who went to the polls in 2008 voted for President Barack Obama.

But it’s unlikely those voters would be more likely to support Romney merely because he has a Latino running mate, said Matt Barreto of Latino Decisions, which conducted a widely cited poll of Hispanic voters in January for Univision and other media outlets.

"We don’t have any evidence that he would provide any significant boost to Romney if he were on the ticket," Barreto said, noting that the poll found that Rubio did best in Florida with first- and second-generation Cuban-Americans, but was less popular with Hispanic voters who had roots in Puerto Rico, Colombia and Mexico.

"He’s not going to be the type of candidate who can go out and resonate with the Mexican-American audiences in the Southwest," Barreto said.

Outside of Florida, Rubio may be gaining popularity but still isn’t that well-known yet.

A Quinnipiac University poll released Thursday found that voters in the swing states of Ohio, Florida and Pennsylvania were far more likely to support their hometown politicians as vice presidential contenders. In Florida, they support Rubio. In Ohio, it’s Republican Sen. Bob Portman. And in Pennsylvania, voters like neighboring New Jersey Gov. Chris Christie the most as a possible VP.

It’s not clear how much Rubio helps Romney in Florida, either. A mid-April poll by Public Policy Polling of North Carolina found that with Rubio on the ticket, Romney drops in Florida from 45 percent to 43 percent. Obama stays at 50 percent, PPP pollster Tom Jensen wrote.

Among Hispanics in Florida, the pollsters found that Obama leads 52 percent to 37 percent with Hispanics. With Rubio on the ticket, Obama still leads 52 to 37 with Hispanics.

George W. Bush garnered 40 and 44 percent of the Hispanic vote nationally in 2000 and 2004.

Whether Romney’s potential vice-presidential running mates are likely to appeal to Hispanic voters or not, they’ll get a more thorough vetting this year than in 2008, thanks to Arizona Sen. John McCain’s last-minute pick of then-Alaska Gov. Sarah Palin.

The Florida Democratic Party has been salivating over the possibility of such a high-profile target as Rubio, releasing daily rundowns of news reports about the senator. They include rehashing Rubio’s personal use of the Republican Party of Florida’s credit card, campaign finance irregularities and his friendship with U.S. Rep. David Rivera, R-Fla., whom state officials cleared of wrongdoing despite a scathing investigative report of his own campaign-finance practices.

Rubio has been on an aggressive public relations push, giving speeches on foreign affairs and appearing on television regularly. He plans to release his memoir June 19, the same date that a Washington Post reporter will publish a biography on him.