CEDAR RAPIDS, Iowa – The Republican presidential candidates on Saturday seized on the first-ever downgrade of the nation’s credit rating as a new line of criticism against President Obama, suggesting that ultimate responsibility rested in the Oval Office even though the rating agency, Standard & Poor’s, cited the overall political gridlock in Washington as a major cause for its decision.

“It happened on your watch, Mr. President,” Michele Bachmann, the Minnesota representative and Tea Party caucus leader, said, drawing applause at an afternoon rally here. “You were AWOL. You were missing in action.”

She criticized the president for spending the day at Camp David.

“I’m calling on the president of the United States to come back to the White House,” Mrs. Bachmann said, “address the American people before the markets open on Monday and give us his positive plan for putting the ratings back up to the AAA rating.”

The political recriminations were swift over the decision by S.& P., one of the major credit rating agencies, to take the unprecedented step of downgrading the credit of the United States government. It became an instant addition to the growing critique being built on the campaign trail against the president by Republicans fighting to win their party’s nomination.

As former Gov. Tim Pawlenty of Minnesota campaigned in Grinnell on Saturday, he declared: “What we should be talking about is downgrading Barack Obama from president of the United States.”

Former Gov. Mitt Romney of Massachusetts, who arrives in Iowa next week for a Republican presidential debate, said in a written statement that the credit rating decision “is a deeply troubling indicator of our country’s decline under President Obama.”

The decision by S.& P. became a central part of Mrs. Bachmann’s presentation on Saturday as she met voters in Cedar Rapids and later in Waterloo, the city of her childhood, where she announced her candidacy in June. She warned voters that the credit downgrade “means higher interest rates for everyone.”

“Last night, we had our day of reckoning,” Mrs. Bachmann said. “Eventually it hits the fan.”

In Congress earlier this week, she was among the House Republicans who voted against raising the debt ceiling and the budget compromise that was negotiated by Speaker John A. Boehner and Republican leader in the Senate, Mitch McConnell. She singled out Mr. Obama for criticism, but she said there was plenty of blame to go around.

“The responsibility is of those in Washington, D.C., who put the deal together,” Mrs. Bachmann said in a brief exchange with reporters. “But the real problem in all of this is that President Obama has failed to give leadership on this issue.”