Updated: July 13, 6:42 p.m.

Days after ripping Republicans for the political theater of holding a health care repeal vote, Democrats are planning their own bit of campaign messaging: a new push for legislation to force disclosure of political spending.

The Senate is expected to vote Monday night on whether to move forward with a bill to do just that, but Republicans are expected to lead a filibuster of the measure, as they have in the past. In fact, Democratic sources said they will have a majority for the bill but that they expect to be well short of the 60 votes needed to cut off debate. They also expect to lose one or two Members of their caucus on the vote.

Undeterred by the bill’s poor prospects, Rep. Chris Van Hollen (D-Md.), the sponsor of the House version of the bill, filed a discharge petition this week with hopes of forcing a House vote as well.

“This DISCLOSE Act will shine its light on the sources of the secret money polluting elections,” said Sen. Sheldon Whitehouse (D-R.I.), who was joined by Senate Rules and Administration Chairman Charles Schumer (D-N.Y.) and several colleagues at a Thursday press conference.

Republicans fired back, saying that the legislation would allow unions to escape disclosure requirements, meaning that, for now, the legislation isn’t going anywhere.

“This ‘new’ proposal once again disproportionately favors unions and is nothing more than a poorly veiled attempt to stifle political speech — Democrats’ strategy to win elections. Unfortunately, the Democratic leadership has delivered a Trojan horse,” said Rep. Dan Lungren (R-Calif.), chairman of the House Administration Committee.

But Van Hollen said the law could help Democrats fight “corporations that move jobs overseas to special tax havens, like the Cayman Islands, like Switzerland.” President Barack Obama’s campaign has attacked presumptive GOP presidential nominee Mitt Romney on outsourcing and for holding bank accounts in places such as the Cayman Islands and, at one point, Switzerland.

Still, Democrats say they’ve taken pains to ensure the bill does not exempt any left-leaning political groups, including removing several provisions that had drawn scrutiny.

“I have looked at this bill repeatedly, and there simply isn’t in it a separate standard for unions,” Whitehouse said. He added that he has “repeatedly invited” Republicans to identify the problem in the bill and to propose language that would fix it.

Also missing from the new version of a bill is a carefully crafted exemption for the National Rifle Association that appeared in a 2010 proposal. The gun rights group attacked the Senate version of the bill in a letter Thursday announcing its opposition.

The legislation — called the Democracy Is Strengthened by Casting Light On Spending in Elections, or DISCLOSE Act — requires any covered organization that spends $10,000 or more on a campaign to identify its donors of $10,000 or more in that election. Covered groups include corporations, unions, super PACs and social welfare nonprofits.

A broader version of the measure won House approval in the previous Congress but fell just short in the Senate, with not a single Republican backing the bill. Reform advocates on and off Capitol Hill have lobbied a handful of targeted Republicans to back the effort this time.

The Progressive Change Campaign Committee, a liberal advocacy group, has generated hundreds of calls to Senate offices and launched an online advertising campaign targeting several Senate Republicans leading up to Monday’s vote. Ads were aimed at GOP Sens. Tom Coburn (Okla.), Susan Collins (Maine), John McCain (Ariz.), Lisa Murkowski (Alaska), Dick Lugar (Ind.) and Olympia Snowe (Maine).

But despite talks between McCain and Whitehouse, the likelihood that any Senate Republican will back the bill remains virtually nil. Senate Minority Leader Mitch McConnell (R-Ky.), a vocal opponent of the bill who has said it would invite donor harassment and intimidation, delivered a strongly worded warning to Republicans in an American Enterprise Institute speech last month.

“I know that as November draws near, some of those running for office will feel the need to choose their battles,” McConnell said at the AEI. “There will be a very strong temptation, particularly among conservatives, to take this issue off the table, to make concessions. My advice is to resist the temptation.”

Democrats appear to have nonetheless concluded that assaults on political money will score political points, despite the issue’s failure to help them in 2010. Said Schumer on Thursday: “If there’s one silver lining to this rotten cloud, it’s that it’s focusing the public’s attention on the horrible state of campaign finance. And we believe this offers a fresh opportunity for reform.”

Republicans say unions could escape the disclosure requirements in the bill because of its $10,000 threshold for political expenditures that triggers the reporting requirements.

Some unions have thousands of local affiliates, which could each spend less than $10,000, shielding them from the law, the GOP said.

In an interview, Van Hollen said the argument was “hogwash” and challenged Republicans that if their concern was in good faith, he would co-sponsor an amendment to set the expenditure threshold to whatever amount they wanted.

“I’ll tell you what, we can make it the Van Hollen-Lungren amendment. Or Lungren-Van Hollen!” he said. “Bring the bill to the floor, offer an amendment to reduce the threshold. I will support it. I will urge my colleagues to support it. We can get this done.”

Lungren has previously advocated increasing the amount that individuals can donate to candidates coupled with near-instant disclosure of the donations.

Although he has not publicly focused on the issue this Congress, his office said Lungren was exploring legislative options.

Michael Steel, spokesman for Speaker John Boehner (R-Ohio), said “the Speaker supports the First Amendment.”

In Citizens United v. FEC, the Supreme Court’s 2010 decision that opened the doors to new types of political spending, justices said requiring disclosure is constitutional.

At the Senate press conference this week, Democrats displayed charts showing sharp growth in the percentage of outside political spending that goes undisclosed. Noting that politically active 501(c)(4) social welfare groups operate outside the campaign disclosure rules, Schumer singled out the conservative nonprofit Crossroads GPS for special criticism. The big-spending group is also the target of a recent Federal Election Commission complaint by President Barack Obama’s re-election campaign.

“The group does not have to disclose a single one of its donors, many of whom are giving millions of dollars at a time,” Schumer said.

Whitehouse, though, acknowledged at the press conference that Democrats, too, are raising unrestricted money but said “it would be foolish for the Democratic Party to disarm unilaterally.”

Emily Pierce contributed to this report.