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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 McConnellGOP senator: Don't expect Trump to 'have your back' on healthcare vote Senate Dems step up protests ahead of ObamaCare repeal vote Johnson becomes fourth GOP senator unwilling to proceed on healthcare bill 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 JohnsonJohnson becomes fourth GOP senator unwilling to proceed on healthcare bill Overnight Healthcare: CBO score imperils ObamaCare repeal | Breaking down the numbers | WH hits back over score | Trump phones holdouts | Dems plan floor protest New CBO analysis imperils GOP ObamaCare repeal MORE (R-Okla.), Rand PaulRand PaulJohnson becomes fourth GOP senator unwilling to proceed on healthcare bill Five takeaways from the CBO score on Senate ObamaCare bill Overnight Healthcare: CBO score imperils ObamaCare repeal | Breaking down the numbers | WH hits back over score | Trump phones holdouts | Dems plan floor protest MORE (R-Ky.) and Mike LeeMike LeeFive takeaways from the CBO score on Senate ObamaCare bill Overnight Healthcare: CBO score imperils ObamaCare repeal | Breaking down the numbers | WH hits back over score | Trump phones holdouts | Dems plan floor protest New CBO analysis imperils GOP ObamaCare repeal 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.