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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 McConnellSenate panel votes to confirm Trump’s Transportation pick Sanders set for clash with Trump’s budget pick Cabinet picks boost 2018 Dems 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 Healthcare: GOP governors defend Medicaid expansion GOP senator: Let's work with Dems to 'fix' ObamaCare Right renews push for term limits as Trump takes power MORE (R-Okla.), Rand PaulRand PaulTrump's CIA chief clears Senate Overnight Defense: Trump nominates Air Force secretary | Senate clears CIA director | Details on first drone strike under Trump Dems blast Trump plans for deep spending cuts MORE (R-Ky.) and Mike LeeMike LeeOvernight Finance: Trump takes US out of Pacific trade deal | WH says Trump has left his businesses | Lobbyists expect boom times GOP senator floats eliminating the corporate income tax Booker is taking orders from corporate pharmaceuticals 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.