Correction Appended

For as long as we can remember, Republicans opposed to campaign finance reform — supposedly on free-speech grounds — claimed that they wholeheartedly endorsed full and fast disclosure of contributions and expenditures as the surest protection against corruption.

But now that the Supreme Court’s conservative majority has opened the way for unlimited corporate spending on campaigns, suddenly Republicans are backtracking on disclosure, claiming it will “chill” free speech.

It’s rank hypocrisy, and those who indulge in it ought to be ashamed of themselves.

We’re likely to see the spectacle unfold on the House floor when the flawed DISCLOSE Act (for “Democracy Is Strengthened by Casting Light on Spending in Elections”) comes up for debate.

Senate Republicans are threatening to filibuster it.

The measure, of course, is a response to the Supreme Court’s decision in Citizens United v. Federal Election Commission striking down more than a century of fitful effort to limit corporate influence on political campaigns and more than a half-century of similar action directed at labor unions.

Under the ruling, corporations and unions still are prohibited from making direct contributions from their treasuries to candidates but can spend any amount they want on independent ads supporting or opposing candidates.

We agree with critics — including the White House and four members of the Supreme Court — who contend that the decision was an outrageous display of conservative judicial activism. Polls indicate that huge majorities of voters oppose the decision.

But in the ruling, the court reaffirmed that requirements for disclosure of those funding campaign ads were constitutional, along with disclaimers by the sponsor that he or she backed the ad.

Disclaimers “provide information to the electorate,” the court majority said, citing previous decisions, and “insure that voters are fully informed about who is speaking.” Disclosure “is the less-restrictive alternative to more comprehensive speech regulations.”

Disclaimers and disclosure are the essence of the DISCLOSE Act, requiring that corporations — for-profit and nonprofit — disclose their participation in campaign ads to the FEC and that the CEO appear in the ad endorsing it.

The measure is now flawed because it contains disclosure exemptions for donors to some special interests. First, the National Rifle Association got an exemption when it threatened to kill the bill. Then other loopholes were opened for groups such as AARP and the Sierra Club, nonprofits organized under Sec. 501(c)(4) of the tax code.

Proposals have been made to exempt all such nonprofits, but that would open the door for corporation- and union-backed front groups spending millions without disclosing their sponsors.

Republicans such as Senate Minority Leader Mitch McConnell (Ky.), long an opponent of campaign finance reform and a one-time staunch advocate of disclosure, are denouncing the exemptions as “backroom dealing.”

But backroom deals would not be necessary to pass the bill if the GOP held true to its original position. As matters stand, partial sunlight is better than none.

Correction: June 23, 2010

The editorial incorrectly described the tax status of AARP and the Sierra Club, which would get exemptions under the latest version of the DISCLOSE Act. They are 501(c)(4) organizations.