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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 McConnellThe Memo: Winners and losers from the battle over health care GOP senators pitch alternatives after House pulls ObamaCare repeal bill Under pressure, Dems hold back Gorsuch support 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 JohnsonLawmakers share photos of their dogs in honor of National Puppy Day GOP targets Baldwin over Wisconsin VA scandal The Hill's Whip List: 36 GOP no votes on ObamaCare repeal plan MORE (R-Okla.), Rand PaulRand PaulGOP senators pitch alternatives after House pulls ObamaCare repeal bill Rand Paul takes victory lap on GOP health bill Paul: Pence should oversee Senate ObamaCare repeal votes MORE (R-Ky.) and Mike LeeMike LeeGOP senators pitch alternatives after House pulls ObamaCare repeal bill How 'Big Pharma' stifles pharmaceutical innovation Overnight Finance: Senators spar over Wall Street at SEC pick's hearing | New CBO score for ObamaCare bill | Agency signs off on Trump DC hotel lease 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.