Sen. Bob Casey announced Wednesday his support of a bill that would compel donor disclosure by any organization spending more than $10,000 on electioneering. His co-sponsorship came the same day as Citizens United – which successfully petitioned the Supreme Court to roll back contribution limits – endorsed Casey opponent Tom Smith.

“The Citizens United decision has led to a flood of corporate money into campaigns, giving the very wealthy the ability to saturate the airwaves anonymously without being accountable for their words,” said Casey. “This is commonsense legislation to respond to the increased power of corporate special interests and the role of money in politics.”

Supporters note that Republicans in general, and members of GOP leadership specifically, have voiced support for disclosure measure in the past. Thus, they say, it’s a non-partisan issue.

That’s because, practically speaking, more of the secret money Casey and his 33 Democratic colleagues in the Senate want to disclose has gone to Republicans than Dems.

National Democrats largely avoided independent political groups in protest in 2010, Republican donors flocked to them – to devastating effect.

Earlier this year President Barack Obama – an outspoken critic of the Citizens United decision – announced his approval of a similar group founded by his former advisers and aimed at boosting his re-election effort.

That’s not the reason Casey is supporting the measure, says spokesman Larry Smar.

“The real beneficiaries are the American people. The DISCLOSE Act will shed light on the shadowy groups trying to influence elections. Greater transparency and disclosure in elections is good for democracy and for the country.”

But that’s the reason two of his opponents said they oppose it.

“This is another example of grossly one-sided partisan ‘feel good’ legislation,” said Peter Towey, campaign manager to Steve Welch. “When Bob Casey agrees to apply the same rules to his labor union bosses, then we’ll know that he’s serious about financial disclosure.”

Welch, an entrepreneur from Chester County, is seeking to unseat Casey. He has been endorsed by the Pa. Republican state committee.

Tom Smith, a former Tea Party leader and coal company owner, agreed.

“In the United States Senate, I will fight to protect freedom of speech and individual privacy.  Of course Senator Casey would support a bill that limits the free speech of his political opponents while providing a giant loophole for his liberal union boss friends.”

Supporters note that the legislation applies the same standards to labor unions as any donors. The proposal would require “any covered organization” that spends at least $10,000 to file a report with the Federal Election Commission within 24 hours, including among other things the names of all of its donors who gave $10,000 or more.

Additionally, the legislation includes disclaimer requiring any political advertisers to list its top funders in the ads. The head of the organization also must appear in the ad and state that he or she approves the message.

Democrats note that it’s not a neutral issue for the GOP, either. If either man hopes to unseat Casey, they will almost certainly need significant spending on their behalf by outside groups.

“With their far right views gaining little traction, it’s not surprising that the Republican Senate candidates are groveling for Karl Rove and corporate special interests to run attack ads for them,” said Mark Nicastre, spokesman for the Pa. Democratic Party.

Case in point: Citizens United. The conservative organization began a new era in campaign spending rules when it won a 2010 case before the Supreme Court.

The ruling resulted in the elimination of contribution limitations to certain organizations. The DISCLOSE Act wouldn’t touch contribution limits, but would instead target 501(c)4 status “social welfare organizations” – the most noteworthy of which being Karl Rove’s Crossroads GPS. Such groups are not required to disclose their donors in most cases.

Citizens United’s PAC endorsed Smith and contributed $10,000 to his campaign.

“I’m honored to have the support of Citizens United Political Victory Fund, and share their conservative vision of reducing the size of government, lowering taxes, cutting spending, and promoting family values,” said Smith in a press release on Wednesday. “I look forward to working with them to defeat the Obama-Casey agenda in November.”