In an abrupt reversal, Senate Minority Leader Mitch McConnell on Monday endorsed a moratorium on earmarks that GOP conservatives are seeking to send a signal that the Republican Party is serious about curbing federal spending.

The Kentucky lawmaker announced his concession on the Senate floor, moments after the start of a lame-duck session of Congress that could stretch into mid-December. McConnell's announcement served as recognition that his bid to retain the practice of steering money to pet projects was losing steam to an emboldened coalition of tea party-backed senators, led by Sen. Jim DeMint (R-S.C.).

"Make no mistake, I know the good that has come from the projects I have helped support throughout my state. I don't apologize for them," McConnell said. "But there is simply no doubt that the abuse of this practice has caused Americans to view it as a symbol of the waste and the out-of-control spending that every Republican in Washington is determined to fight. And unless people like me show the American people that we're willing to follow through on small or even symbolic things, we risk losing them on our broader efforts to cut spending and rein in government."

In a statement, President Obama said he embraced McConnell's decision.

"I welcome Senator McConnell's decision to join me and members of both parties who support cracking down on wasteful earmark spending, which we can't afford during these tough economic times," Obama said. "As a Senator, I helped eliminate anonymous earmarks, and as President, I've called for new limitations on earmarks and set new, higher standards of transparency and accountability."

According to the nonprofit group Taxpayers for Common Sense, McConnell has been one of the most prolific beneficiaries of the earmark system. In the three years since public disclosure of earmark requests was required, the former member of the Appropriations Committee has sought nearly $1 billion worth of earmarks, mostly benefiting his home state.

McConnell defiantly guarded the earmarking practice as recently as last week, arguing that eliminating earmarks doesn't reduce spending, but rather redirect it. But few of his colleagues were willing to stand alongside him against tea party groups and conservative leaders outside of Congress who have elevated the earmark debate to a litmus test for fiscal restraint.

By late Monday, 27 Republican senators and senators-elect had gone on record supporting the ban.

Obama also has supported an earmark moratorium and has encountered similar resistance from veteran Democratic lawmakers, who like McConnell are former appropriators. In the Senate, the GOP earmark moratorium amounts to a non-binding party rule, because Democrats remain the majority party. But in the House, Republicans can officially eliminate the practice when they take control of the chamber next year.

In one sign that McConnell was losing the battle, earmark opponent Tom Coburn (R-Okla.) announced Monday that he would force the Senate to hold a public vote on an earmark moratorium as an amendment to the first bill to move through the chamber this week. Republicans had planned to vote on earmarks by secret ballot at a closed-door meeting on Tuesday.

"The American people have sent a clear message that it is time for this body to make hard choices and live within our means," Coburn said in a statement. "Imposing a moratorium on earmarks is an important step that will - as a matter of symbol and substance - begin a new era of sobriety in Washington. America did just fine for 200 years without earmarks, and Congress will do just fine without them."

murrays@washpost.com Staff writers Felicia Sonmez and Paul Kane contributed to this report.