Barack Obama has been rocked by a resignation from his government less than 24 hours after he pushed through his economic stimulus package.

Republican senator Judd Gregg, who was chosen to be commerce secretary, quit over "irresolvable differences" on money to be used to rescue to the US economy.

His nomination was announced only a week ago, and he becomes the fourth member of the president's team to step down in the first month of his presidency.

Mr Obama had originally picked New Mexico Governor Bill Richardson for the commerce post, but he withdrew over an investigation into campaign donations.

White House press secretary Robert Gibbs said Mr Gregg had been unable to support some of the president's "key economic priorities".

Republicans have been opposed to the rescue package, saying it contains too much wasteful spending and not enough tax cuts.

Despite fierce opposition, the Senate and House of Representatives approved the package and Mr Obama is expected to sign it into law within days.

The president did not comment on the resignation, but a White House spokesman said Obama regretted the "change of heart".

Despite Mr Obama's relief that the $789 billion stimulus package has been approved, Mr Gregg's resignation blows a hole in any hopes he had of bipartisan support for his administration.

Mr Gregg said in a written statement that he had been honoured by his selection and that he admired President Obama's "willingness to reach across the aisle".

But he said he had found there were "irresolvable conflicts" on issues, pointing to the economic stimulus package and the country's census.

"Prior to accepting this post, we had discussed these and other potential differences, but unfortunately we did not adequately focus on these concerns," the statement said.

"We are functioning from a different set of views on many critical items of policy."

The 62-year-old senator for New Hampshire said that he hoped to help Mr Obama more by remaining in the Senate.

His resignation follows that of Tom Daschle as health secretary and Nancy Killefer as chief performance officer.

Both stepped down after unresolved tax issues became public and Mr Obama admitted he had "screwed up" with his choices.