Rubio raises more than $200k for McConnell

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 McConnellCures bill in jeopardy amid drug pricing push Senate Democratic super PAC sets fundraising record Five takeaways from Florida Senate debate 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 JohnsonSenate GOP pounces on FBI's Clinton probe Dems back in Wisconsin with M ad blitz Democrats pounce on Cruz's Supreme Court comments MORE (R-Okla.), Rand PaulRand PaulWhat the 'Bernie Sanders wing of the GOP' can teach Congress GOP senators avoid Trump questions on rigged election How low is the bar for presidential candidates, anyway? MORE (R-Ky.) and Mike LeeMike LeeDonald Trump's Mormon PR problem Trump's big worry isn't rigged elections, it's GOP establishment GOP senators avoid Trump questions on rigged election 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.