Sen. John Thune (R-S.D.) speaks on Capitol Hill on Oct. 12. (Mark Wilson / Getty Images )
Mitt Romney has earned the endorsement of a leading Republican who himself was thought to be presidential timber, South Dakota Sen. John Thune.
Thune's support could boost Romney's campaign in the leadoff caucus state in Iowa, where the former Massachusetts governor has not campaigned as aggressively as his chief rivals.
Thune will join Romney on Wednesday morning at a stop in Des Moines and participate on a tele-town hall meeting with supporters statewide in the afternoon.
"Mitt Romney has shown throughout his life in the private sector, as leader of the Olympics, as governor, and in this campaign that he will not back down from difficult challenges," Thune said in a prepared statement.
"Sen. Thune has been a leading voice in the Senate. He will be a trusted advisor as I bring this message to voters, work to reverse President Obama’s failed policies," Romney said.
Thune rose to prominence when he defeated the Democratic leader of the Senate, Tom Daschle, in a contentious 2004 campaign. He was unopposed for reelection in 2010, and thought to be considering a 2012 presidential run.
He ruled out a national campaign in February but is sure to be among the Republicans on a short list for vice president.