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Rubio's help on the trail should boost McConnell's credibility with Tea Party voters in Kentucky, a group that's long been wary of McConnell's record in the Senate. Some Tea Party groups have been actively pursuing a primary challenger to McConnell, but no possible contenders have yet jumped into the race.

"Mitch McConnellMitch McConnellPresident Obama should curb mass incarceration with clemency Cruz, DeSantis to introduce constitutional amendment on term limits Democrats back down from shutdown threat MORE's quiet, principled leadership is key to everything we accomplish on Capitol Hill," Rubio said during the event, according to CNN.

"The way he fought ObamaCare, tooth and nail, and forced the Democrats to use extraordinary measures to pass it was a critical catalyst in electing my entire class of Senate conservatives in 2010. Without Mitch, I don't know that we'd have Senator Rubio, Senator Johnson, Senator Paul or Senator Lee."

Sens. Ron JohnsonRon JohnsonOvernight Regulation: Obama's reg czar under pressure | Fight looms over Trump EPA pick Obama's regulatory czar under pressure to cutoff 'midnight rules' Week ahead: GOP quickly laying groundwork for reg rollback MORE (R-Okla.), Rand PaulRand PaulTrump stumps for Louisiana Senate candidate ahead of runoff Giuliani won't serve in Trump administration Will justice in America be Trumped? MORE (R-Ky.) and Mike LeeMike LeeSenate sends annual defense bill to Obama's desk Overnight Healthcare: Medical cures bill finally heads to White House Overnight Energy: Trump taps EPA foe to head agency | Energy reform bill officially dead MORE (R-Utah) all have Tea Party support, and affiliating himself with all three will likely help McConnell appeal to those skeptical voters on the right.

McConnell is likely to face a strong challenge from Democrats as well, who believe he is one of the nation's most vulnerable incumbents in 2014. Two Democratic contenders, actress Ashley Judd and Kentucky Secretary of State Alison Lundergan Grimes, have emerged as possible challengers.