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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 McConnellMcConnell: Trump's speech should be 'tweet free' Protesters crash McConnell's speech The Hill's 12:30 Report 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 JohnsonA guide to the committees: Senate Hopes rise for law to expand access to experimental drugs Dems ask for hearings on Russian attempts to attack election infrastructure MORE (R-Okla.), Rand PaulRand PaulRand Paul rejects label of 'Trump's most loyal stooge' GOP healthcare plans push health savings account expansion Congress must reform civil asset forfeiture laws MORE (R-Ky.) and Mike LeeMike LeeLessons from the godfather of regulatory budgeting Congress must reform civil asset forfeiture laws A guide to the committees: Senate 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.