Senate Majority Leader Harry Reid (D-Nev.) is hosting a fundraiser for Alison Lundergan Grimes, Senate Minority Leader Mitch McConnell's (R-Ky.) Democratic opponent.
Though McConnell pledged not to campaign against Reid during his 2010 race, he also shifted course and helped Reid's GOP opponent, Sharron Angle, raise money.
In 2004, GOP Senate Leader Bill Frist (R-Tenn.) campaigned for John Thune (R-S.D.) against then-Democratic Leader Tom Daschle (D-S.D.).
The fundraiser, first reported by Kentucky news outlet Channel 2, is scheduled for Oct. 11 at a McCormick & Schmick's restaurant in Las Vegas, and is organized by the Searchlight Leadership Fund, an organization Reid founded.
Tickets range from $1,000 for an individual to $5,000 for a political action committee co-host.
The Kentucky Republican Party pounced on the fundraiser. Spokeswoman Kelsey Cooper drew ties between the candidate and Reid's positions on coal.
"It should tell Kentuckians all they need to know that the biggest supporter of Alison Lundergan Grimes' campaign is Harry Reid, the man who thinks 'coal makes us sick.' If Alison was elected to the Senate, it would ensure that liberal Harry Reid and Barack Obama continue to set national coal policy rather than a Kentucky champion for coal, Mitch McConnell," Cooper said in a statement.
Coal mining is a big industry in Kentucky, and Republicans believe Lundergan Grimes may be vulnerable because of Democratic-backed policies to curb emissions from coal-fired power plants, which opponents say will hurt the coal industry.
The Lundergan Grimes campaign and Reid's office didn't respond to requests for comment.