WASHINGTON (AP) — Senate Republicans blocked Democratic-backed legislation requiring organizations pouring hundreds of millions of dollars into campaign ads to disclose their top donors and the amounts they spend.

GOP opposition prevented Democrats from getting the 60 votes needed to bring what is known as the Disclose Act to the Senate floor. The vote was 51-44.

Democrats revived the act during a presidential election campaign in which political action committees and nonprofit organizations, funded by deep-pocketed and largely anonymous contributors, are dominating the airwaves with largely negative political ads.

Another version of the Disclose Act passed the then-Democratic-controlled House in 2010 but was similarly blocked by Republicans in the Senate. Republicans cite First Amendment rights and say the bill favors unions in opposing the legislation.

In a statement late Monday, President Barack Obama said he was disappointed and chided Republicans for blocking the bill.

"Instead of standing up for the American people, Republicans stood with big banks and oil companies — special interests that certainly don't need more clout in Washington," Obama said. "I'm disappointed Republicans in Congress failed to take action and hold corporations and special interests accountable to the American people."

Senate Republican Leader Mitch McConnell of Kentucky accused Democrats of wasting time on bills "they know won't pass but which give them a chance to make a fuss about a problem that doesn't exist and blow a kiss to the unions for good measure."

The bill, which would not have gone into effect until next January, would have required any organization that spends $10,000 or more during an election cycle to file a report within 24 hours identifying any donors who gave $10,000 or more. Current election law requires super political action committees, or PACS, to make periodic reports to the Federal Election Commission, but nonprofit groups, including social welfare organizations, labor unions and trade groups, generally do not have to reveal the sources of election-related spending.

"Perhaps Republicans want to shield the handful of billionaires willing to contribute nine figures to sway a close presidential election," said Senate Majority Leader Harry Reid of Nevada. He said this election was in danger of being bought by "17 angry, old, white men."

The White House, in a statement, said the bill was needed so Americans would "know who is attempting to influence the nation's elections." Without the bill, it said, "corporations and wealthy individuals will continue to be able to shield their donations from disclosure."

Democrats have been pushing for more disclosure since the Supreme Court's 5-4 ruling in the 2010 Citizens United case that overturned a decades-old law barring corporations, unions and other organizations from spending on advertising and other forms of political activity.

Sen. Sheldon Whitehouse, D-R.I., said super PACS and other outside groups have spent some $150 million during this election cycle, double the amount spent during the same period in 2008. Election campaigns, he said, are "now waged by shadowy political attack groups posing as social welfare organizations."

The Sunlight Foundation watchdog group said Monday that Crossroads GPS, a conservative organization formed by Republican strategist Karl Rove, has announced ad buys so far totaling $83 million while Americans for Prosperity, another conservative group founded by brothers Charles and David Koch, have bought ads worth $32 billion.

Groups such as Crossroads GPS, are classified as social welfare organizations not subject to disclosure laws as long as they spend more than half their resources toward "social welfare" purposes. But that 50 percent can go for "public education" that can be highly political in nature.

Knowing they were facing defeat, Democratic supporters announced earlier in the day that they would hold a "midnight vigil," speeches going late into the evening on the need for greater campaign transparency. "We can't let the special interests off the hook after just one round," Whitehouse said.

Monday's vote was strictly along party lines except for Reid, who changed his vote to "no" in a procedural move that allows him to bring up the legislation again on Tuesday.