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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 McConnellAddison (Mitch) Mitchell McConnellMcConnell expects Paul to return to Senate next week Former Hill staff calls for mandatory harassment training Gaming the odds of any GOP tax bill getting signed into law 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 JohnsonRonald (Ron) Harold JohnsonDemocratic Homeland Security members request additional DHS nominee testimony Key differences between the Senate and House tax plans Senate panel delays vote on Trump’s Homeland Security pick MORE (R-Okla.), Rand PaulRandal (Rand) Howard PaulGOP senator asks to be taken off Moore fundraising appeals Red state lawmakers find blue state piggy bank Prosecutors tell Paul to expect federal charges against attacker: report MORE (R-Ky.) and Mike LeeMichael (Mike) Shumway LeeOvernight Health Care: Trump officials to allow work requirements for Medicaid GOP senator: CBO moving the goalposts on ObamaCare mandate Cornyn: Senate GOP tax plan to be released Thursday 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.