WASHINGTON -- Senate Republicans beat back Democratic attempts Tuesday to pass a bill that would impose stringent disclosure requirements on corporations, unions and other independent groups that finance ads for political campaigns.

Ignoring pleas for passage from President Barack Obama, not a single Republican voted for a motion to limit debate and proceed toward final passage of the so-called DISCLOSE Act. Without Republican support, the debate-limit motion failed 57-41, falling short of the 60 votes required under Senate rules to shut off debate.

The failure puts the entire bill in jeopardy, though Democratic leaders could seek to revive it later.

Senate Democrats even made late changes to the bill -- removing a provision that Republicans perceived as union-friendly -- in an unsuccessful attempt to lure Sens. Scott Brown, R-Mass., Susan Collins or Olympia Snowe, both R-Maine, to their side.

Senate Majority Leader Harry Reid of Nevada, was the only Democrat to vote with Republicans, a tactical move that will let him bring up the bill again at a later date.

"The message we have is this is the first battle, but not the end of the war," said Sen. Charles Schumer, D-N.Y., the bill's author, shortly after the vote. "We will keep fighting and fighting and fighting until this passes, until we get that one courageous Republican. ..."

Republicans argued that the bill is unconstitutional because it would impose a restriction on free speech guaranteed by the First Amendment. They also said the measure's true purpose was to stifle the flow of campaign ads against vulnerable Democratic incumbents in November's elections.

"The bill we're about to vote on here ... is about jobs, all right. It's about saving Democratic senators' jobs," Senate Minority Leader Mitch McConnell, R-Ky., told reporters.

The Senate bill would require most independent groups, including labor unions and corporations, to disclose the names of the top five donors whose money helped fund political ads.

It also would require corporate and union executives to appear in political ads that their organizations help pay for and state that the executive "approves this message," as candidates now do in campaign commercials. The bill also would prevent use of federal TARP money in elections and curb foreign nationals and countries from contributing to campaigns.

The House version passed on a 219-206 vote last month.

Democrats crafted the DISCLOSE Act after the Supreme Court's 5-4 decision in January -- the Citizens United vs. Federal Election Commission -- which struck down decades-old laws barring corporations and unions from directly supporting campaigns.

Washington correspondent Daniel Malloy writes the "Pittsburgh On The Potomac" blog exclusively at PG+, a members-only web site of the Pittsburgh Post-Gazette. Our introduction to PG+ gives you all the details.