The 57-41 vote was three short of the total needed to advance the measure, which calls for greater disclosure on campaign advertising funded independently by corporations, unions and other organizations, but included an exemption for the National Rifle Association and a small number of other groups.
Less than 100 days before the elections, the debate was highly political - and the outcome widely anticipated.
Anticipating defeat, Democrats swiftly unleashed a coordinated attack employing one of their emerging campaign themes. "After a year of defending big banks, big insurance, big oil and other special interests, Republicans might want to drown out the voices of Americans who don't have the financial resources of big corporations but want to have their say in this year's elections," the party's chairman, Tim Kaine, said in a statement.
Republicans, anticipating big gains in the fall, folded the day's Senate events into their own election-year argument - that Democrats have been unsuccessful in easing double-digit unemployment.
"Today was a rebuke to congressional Democrats who need to put aside their electoral self-interest and start addressing our struggling economy, which continues to be the primary concern among American voters," GOP Chairman Michael Steele said.
Democrats drafted the bill in response to a Supreme Court ruling last winter that said corporations and unions were free to spend their own money on advertising, mass mailings and other forms of political activity. A companion measure cleared the House last month on a near party-line vote over vociferous Republican protests.
Under both bills, nearly all organizations airing political ads independently of candidates or the political parties would be required to disclose their top donors and the amounts they paid. The group's CEO or other top official would be required to appear on screen taking responsibility for the commercial.
Additionally, any business, union or other entity holding a government contract worth more than $10 million would be banned from a variety of political activity, as would firms in possession of federal bailout funds and corporations in which foreigners own more than a majority of voting shares.
Corporations, labor unions and others engaging in certain types of independent political activity would be required to report donations, dues or other contributions from all donors who have given $600 or more.
Whatever the partisanship involved, the legislation was a case study in the power of special interests.
The NRA exemption originally was added in the House after Democrats concluded it was essential if moderate and conservative members of the rank and file were to vote for the legislation. The loophole was then broadened as the leadership sought to quell criticism that it was bowing to a group that had been instrumental in blocking gun control bills long sought by liberals.
In the end, the NRA was officially neutral, angering Republicans who didn't want Democrats to have any political help in advancing the legislation.
The U.S. Chamber of Commerce and AFL-CIO both opposed the bill, a rare agreement of two arch-foes. Organized labor had been neutral when the bill passed the House, but switched its position after changes were made to tighten reporting requirements for transfers of dues money from local unions and their national headquarters.
The American Civil Liberties Union also was against the bill, arguing it was a violation of First Amendment rights.
The constitutional debate was overshadowed by the approach of elections, though.
Sen. Chuck Schumer, D-N.Y., the bill's chief sponsor, said it was intended to crack down on shadowy campaign groups that spend heavily on attack ads. Referring to Republicans, he said, "There are visions, visions in people's heads of Karl Rove spending $50 million funded by people we don't know to attack candidates for reasons we're not sure of and never putting their name to it."
Republicans were unpersuaded.
"This bill is a partisan effort, pure and simple, drafted behind closed doors by current and former Democrat campaign committee leaders," said Sen. Mitch McConnell, the GOP Senate leader. "And it's aimed at one thing and one thing only: this bill is about protecting incumbent Democrats from criticism ahead of November."
Schumer was chairman of the Democratic Senate Campaign Committee, which is responsible for maximizing the party's chances in Senate races, before stepping aside after the 2008 elections.
Rep. Chris Van Hollen, D-Md., the sponsor of the House-passed measure, is chairman of the Democratic Congressional Campaign Committee.
In a statement, he said that "Senate Republicans once again voted to protect powerful special interests instead of Americans - this time voting for what amounts to a corporate takeover of our elections."
"Despite widespread Republican support for increased transparency and disclosure in the past, their opposition to the DISCLOSE Act ensures that voters will be left in the dark as unlimited corporate money is spent by shadowy groups seeking to mislead Americans," he said.
The votes broke down strictly along party lines, although Reid switched to the opposition at the end of the roll call so he could seek a second test vote in the future. Sens. Joseph Lieberman, I-Conn., and John Ensign, R-Nev., were absent.