Senate Minority Leader Mitch McConnell (R-Ky.) said he would support who ever wins the Republican Tuesday between O'Donnell and Rep. Mike Castle (R-Del.).
"The voters of Delaware are going to decide this very day as we are speaking who they want to represent them in the Senate," McConnell said during an appearance on MSNBC. "I'm going to support the nominee."
Castle is facing a last-minute conservative surge from O'Donnell, the Tea Party candidate who's been backed by former Alaska Gov. Sarah Palin (R) and Sen. Jim DeMint (R-S.C.) and his Senate Conservatives Fund.
O'Donnell is the latest insurgent Republican candidate backed by the Tea Party movement to threaten an establishment candidate, following Sharron Angle in Nevada, Mike Lee in Utah and Joe Miller in Alaska.
The outsider candidates have brought new voters to the primaries and generated enthusiasm, but also may be weakening the Republican party's field of candidates for the general election. Castle argues O'Donnell would lose a general election in Delaware, and polls suggest he's correct.
McConnell has made clear that he welcomes members of all ideological stripes to the Senate GOP conference, though the new crop of would-be conservative senators is seen as potentially less loyal to him as GOP leader.
"I'm going to support the Republican nominee for the Senate anywhere in America," he said.
That support extends to Alaska, McConnell said, where he said he supported Miller rather than Sen. Lisa Murkowski (R), the member of the Senate GOP leadership team to lose in that state's primary.
"She knows I'm supporting Joe Miller," McConnell said. "We expect him to be the next senator from Alaska."