US Sen. John Thune won't run against Obama in 2012

But the nuts and bolts of a campaign never seemed to coalesce. During a meeting with New Hampshire activists, he defended specific spending requests, known as earmarks, he secured for his state. Earlier this year, he shook up his fundraising team.

Aides sought to downplay the significance of the move while cautioning that Thune had yet to decide whether to seek the nomination.

With six years each in the U.S. House and Senate, Thune has spent considerably more time in Congress than Obama had when he was elected president. His opposition to gay marriage and abortion has earned him points with evangelicals, while his pro-business, anti-tax and pro gun-rights stances have garnered support among more libertarian leaning conservatives.

But some have criticized Thune's vote for the Wall Street bailout in 2008. Thune acknowledged that vote was "clearly an issue for a lot of conservatives."

With Thune's exit, it appears the most serious White House contenders will be current and former governors who will campaign on their records running statehouses.

Asked if his decision not to run next year was final, Thune demurred.

"I don't think you ever totally rule things out, but I wanted to come to closure on this," he said. "For me, for now, my work is in the United States Senate."

Elliot reported from Washington. Associated Press writer Charles Babington also contributed to this report from Washington.