The decrease could be attributable to a sense that the senator, while popular among conservative activists, has yet to be vetted or tested nationally. Those fears are particularly acute among GOP officials after the selection of Sarah Palin as John McCain's running mate four years ago, a decision that ultimately politically backfired on the Republican presidential nominee.
Still, Rubio's promise remains: He's young, telegenic, from a key state and - most importantly of all - Latino. The likely GOP nominee, Mitt Romney, is none of those things, and has struggled to attract Hispanic support even as the group is poised to play a decisive role in the presidential election.
"[Rubio's] a three-fer: young, Hispanic, and from what may be the most important state - a la 2000," said one Democratic iInsider.
Another Democrat said, "Rubio disrupts Obama's Hispanic strategy in Florida, Arizona, New Mexico, and other states."
He's great on paper, a GOP Insider says. It's just a question if he'll handle the spotlight.
"Hispanic, swing state, conservative," the Republican said. "The only question - is he ready?"
As Rubio's popularity wanes, support for Rep. Rob Portman, R-Ohio, has surged. Barely a consideration six months ago, the former White House budget director was the second-most frequent choice for both sets of strategists - receiving 23 percent support among Republicans and 22 percent for Democrats.
Portman was a key backer of Romney's during the Ohio GOP primary and is considered a safe, well-vetted pick from a crucial swing state.
"Not only is he eminently qualified to be president, but even more importantly no Republican as ever been elected President without carrying the state of Ohio," said one GOP strategist.
Added another Democrat: "It is not about sizzle. Portman would make a great VP - highly qualified, highly competent, and from Ohio."
Outspoken New Jersey Gov. Chris Christie, a Romney supporter, was the third most frequent choice of both parties, receiving 18 percent of the backing from Democrats and 12 percent from Republicans.
"He will make up for the lack of conviction and firm principles on the part of Romney," one GOP insider said. "He'll wipe the floor with Biden in the debates."
Bringing up the rear was Rep. Paul Ryan, R-Wis., who had 8 percent of the Democratic insiders support and 12 percent among Republicans, and Virginia Gov. Bob McDonnell, who had 7 percent backing from Democrats and 8 percent from Republicans.
Louisiana Gov. Bobby Jindal received the least backing among both parties, with only two votes in favor from Republicans and exactly zero from Democrats. He received fewer votes, in fact, the New Mexico Gov. Susana Martinez, who received three write-in votes among Republicans.