Presidential hopeful Sen. Barack Obama (D-Ill.) endorsed a one-year moratorium on Congressional earmarks Monday, only to have his adversary, Sen. Hillary Rodham Clinton (D-N.Y.), join him within hours.

The sight of the two Democratic White House contenders backing an amendment sponsored by Sen. Jim DeMint (R-S.C.) and opposed by Senate leadership prompted DeMint to send out a press release saying he welcomed “growing bipartisan support” for the earmark moratorium. Before the start of the day, DeMint’s effort had attracted only one Democrat and was dismissed by many within his own party.

Obama started the wave by announcing his support and said he would refrain from requesting earmarks this year. “The entire earmarks process needs to be examined and reformed,” he said in a statement. “Over the next year, I hope to work with my colleagues, both Democratic and Republican, to improve the earmarks process.”

Clinton’s office could not be reached for comment.

Senate Majority Leader Harry Reid (D-Nev.) has rejected the moratorium and has defended earmarking. Speaker Nancy Pelosi (D-Calif.) has floated the idea of a moratorium vote on the House side.

Obama and Clinton join their potential GOP rival for the White House, Sen. John McCain (Ariz.) in co-sponsoring the amendment, which would apply block consideration of any appropriation or authorization measure that includes earmarks. Sen. Claire McCaskill (Mo.) was the first Democrat to back the language, joining a small number of Republicans, including Minority Whip Sen. Jon Kyl (Ariz.).

DeMint, who has been pushing his conference to adopt the provision to create “breathing room” in which to overhaul the earmarking process, said he was pleased with his sudden support.

“Earmarks are a bipartisan addiction, and true reform will require bipartisan support. Sen. McCain has worked for years to bring Democrats and Republicans together to close the earmark favor factory, and I’m proud to stand with him. Senators Claire McCaskill and Barack Obama have led their party in this debate and understand we need a timeout on earmarks to truly reform the way we spend taxpayer money. I thank them for their support,” DeMint said Monday before Clinton had joined the effort.

GOP aides said Monday that despite resistance from leadership and the Conference’s Old Bulls, cracks in the opposition to DeMint’s language were beginning to appear, though aides said it is still unlikely that Minority Leader Sen. Mitch McConnell (R-Ky.) — a longtime appropriator and earmark supporter — will move his party behind DeMint. A leadership aide would only say that no decision will be made until after McConnell is able to meet with his leadership team this morning and then poll the conference during its weekly luncheon today.