Sens. Barack Obama (D-Ill.) and Hillary Rodham Clinton (D-N.Y.) have agreed to co-sponsor an amendment establishing a one-year ban on earmarks, which Sen. Jim DeMint (R-S.C.) plans to offer this week to a fiscal 2009 budget resolution. McCain, who has long attacked earmarks as a waste of taxpayer dollars, was one of the original sponsors of the DeMint amendment and has been pushing the Democratic candidates to back the measure.

Obama announced his support for the amendment Monday, which was soon followed by an announcement from Clinton’s office that she would also cosponsor the measure, according to Senate aides.

By sponsoring the measure, the senators can all point to the amendment as an example of their efforts to reform the ways of Washington and to reach across the aisle on difficult issues.

The amendment, which is also sponsored by Obama supporter Sen. Claire McCaskill (D-Mo.), is an example of how this week’s budget debate is likely to devolve into positioning along the campaign trail.

Obama, McCain and Clinton all are expected to return to Capitol Hill for votes on the budget this Thursday. McCain sought no money for earmarks last year, but Obama and Clinton co-sponsored $98 million and $342 million in earmarks respectively last year, according to the Taxpayers for Common Sense.

Senior senators from both parties oppose the amendment, arguing that it is Congress’s prerogative and constitutional right to choose how it should spend federal dollars.

Sam Youngman contributed to the story