After considering a resignation once the U.S. debt deal was reached, Treasury Secretary Timothy Geithner confirmed on Sunday that he will remain at his post -- making him Obama's longest-serving economic adviser after the first-ever U.S. credit downgrade.

"I love my work. And I think if a president asks you to serve, you have to do it," Geithner told NBC/CNBC in an interview.

Geithner has been battling a financial crisis since 2007, working as a top Federal Reserve official and then Treasury secretary. But several developments have made his departure more difficult. The economy has worsened since Congress finally came to an agreement to raise the debt ceiling. And on Friday, Standard & Poor's downgraded the U.S. credit rating for the first time, to AA+ from AAA.

On Friday, Geithner told Obama he would remain in his post, and, hours later, had to take a trip to the White House to inform the president that the nation would likely lost its AAA-credit rating.

"Secretary Geithner has let the president know that he plans to stay on in his position at Treasury," Treasury spokeswoman Jenni LeCompte said in a statement. "He looks forward to the important work ahead on the challenges facing our great country."

White House Press Secretary Jay Carney said:  "The president asked Secretary Geithner to stay on at Treasury and welcomes his decision."

Geithner, who turns 50 later this month, has repeatedly won over Obama in policy debates, shaping the president's response to the financial crisis, and successfully arguing that the government shouldn't seize struggling banks.

But GOP presidential candidate Michele Bachmann has left no questions about her disapproval of Obama's decision to keep Geithner, calling his retention a "tremendous disservice to the American people."

Bachmann Wants Geithner To Go

At a gathering of the Polk County Republican Party in Des Moines, Iowa, Bachmann said that keeping Geithner at the helm of the Treasury was a "bad move" and accused Obama of failing to "inspire confidence in our markets."

"For the sake of the media that are here, please indulge me," Bachmann said before reading a statement.

"The president's refusal to remove Treasury Secretary Geithner shows the president has no plan to restore the AAA-credit rating to the U.S.," she said. "The president is not listening to the people of this country, nor is he providing the leadership that is necessary to bring about economic recovery. I once again, today, in Polk County, Iowa, renew my call for Treasury Secretary Tim Geithner to resign immediately for the sake of our country and to return our economy to full status."