Washington ( CNN ) -- Depending on whom you ask , either Democratic efforts to force big-dollar donors out of the shadows are a smokescreen that hides union clout , or Republicans opposing the efforts are just blowing smoke in an attempt to ensure well-heeled donors stay anonymous . `` Right around an election , when the campaigns start rolling in the money , we 're going to hear about disclosure from whichever side is getting less , '' said Lisa Rosenberg , a government affairs consultant who lobbies for financial transparency for the Sunlight Foundation . `` Whichever party is feeling beaten up is going to say , 'We need to fix it . ' '' Keenly aware that Republican presidential candidate Mitt Romney has out-raised President Barack Obama in direct contributions for the past two months , Democrats twice brought a Senate bill to the floor aimed at unmasking wealthy , secret donors to outside groups -- the kind that have thrown their weight behind Republican candidates this election cycle . Nearly 700 independent political groups have poured more than $ 187 million into 2012 campaigns nationwide so far , according to FEC records compiled by the Center for Responsive Politics . More than $ 41 million of that has come from groups that disclose only a limited amount of information about their donors , or none at all . CNN Money : Top super PAC donors . About $ 140 million of the total has been spent by conservative groups , with most of that laid out during the Republican presidential primary battles , according to the group , which runs the campaign-finance website OpenSecrets.org . It 's Democratic concerns about that kind of money that led to two votes this week on a bill called the Disclose Act , which would have required organizations that spend money on politics to name donors who give more than $ 10,000 . Republicans successfully filibustered the measure both times , with Senate Minority Leader Mitch McConnell accusing Democrats of seeking to cow President Barack Obama 's opponents into submission by making activist groups name their high-dollar donors . `` Democrats can call this bill whatever they want , but they can not conceal its true intent , which is to encourage their allies and discourage their critics from exercising their first amendment right to speak their mind , '' McConnell said . Republicans also complained that the measure gave an unfair break to labor unions , which typically contribute to Democrats . Unions have to disclose their expenditures , but individual members ' dues would most likely fall below the $ 10,000 threshold for disclosure . `` What is the final difference between one $ 10,000 check and 1,000 $ 10 checks ? , '' asked Sen. John McCain , R-Arizona , co-author of a bipartisan 2002 campaign finance law . `` Other than the impact on trees , very little . So why should one be free from having to disclose its origin ? '' Senate Republicans block DISCLOSE Act for second straight day . Sheila Krumholz , executive director of the Center for Responsive Politics , says the power of unions is `` incontrovertible . '' But unions already disclose the amount they spend , and its source -- members ' dues -- is a known quantity , she said . `` What we do n't know is who is bankrolling these patriotic-sounding but vague and unfamiliar groups which are funded to the tune of hundreds of millions of dollars by individuals , by corporations and trade associations , '' she said . `` ... We do n't have good enough and routine enough information about union spending to these outside groups , but we have no information from the other side . '' Labor groups now have to report political expenditures of more than $ 5,000 , though they do n't have to disclose individual members whose dues are pooled to provide that money . Philadelphia labor lawyer Richard Poulson told CNN that 's not a break for the unions , just a reflection of the comparative wealth involved . Opinion : Why Democrats need labor unions . `` The Republicans are essentially crying foul because individual union members are not wealthy enough , '' said Poulson , whose firm of Willig , Williams and Davidson has represented several public- and private-sector unions . `` If we were to go back to square one , I do n't think anyone on labor 's side of the fence is particularly happy the floodgates have been opened with respect to political spending , '' Poulson said . But he added , `` Workers are never going to have as much money as the bosses , and that 's just a fact . And you should n't punish workers for banding together to get some sort of a voice in the process . '' Democrats say legislation is necessary to stanch the tide of money that has been pouring into American politics since the 2010 Supreme Court decision in the Citizen United case . That ruling struck down some long-standing limits on campaign spending by corporations , unions and other groups . A previous version passed the House of Representatives in 2010 , but also fell to a GOP filibuster . Reid : `` ... there is nothing free about an election purchased by a handful of billionaires for their own self-interest . '' `` Instead of standing up for the American people , Republicans stood with big banks and oil companiesâ€ ” special interests that certainly do n't need more clout in Washington , '' President Obama said Monday in a statement released after Republicans successfully blocked the Disclose Act . And Senate Majority Leader Harry Reid said the $ 10,000 limit was added at the request of the National Rifle Association , the GOP-leaning gun-rights lobby . `` We thought that perhaps with the NRA having so much power on the Republicans , that we would raise that to $ 10,000 , which we were told would make them happy , '' the Nevada Democrat said before Tuesday 's vote on the legislation . `` But it did n't make any difference to the Republicans . '' A longtime foe of any campaign finance legislation , McConnell has argued that full disclosure of campaign donations is `` the best disinfectant , '' as he told NBC in 1997 -- but he 's also qualified that call by saying it should be limited to groups directly involved in politics , not `` issue advocacy '' groups like those unleashed by the court 's ruling in the Citizens United case . But the opposition had some high-profile opponents on the left as well . The American Civil Liberties Union opposed both the 2010 and 2012 versions , noting that some features it supported in the previous bill -- such as making corporations disclose their political spending to shareholders and requiring broadcaster to provide low-cost airtime -- were n't included this year . `` Measures intended to root out corruption should not interfere with freedom of expression by those wishing to make their voices heard , and disclosure requirements should not have a chilling effect on the exercise of rights of expression and association , especially in the case of controversial political groups , '' several of the ACLU 's top lawyers wrote in March . And Maine Sen. Susan Collins , a moderate Republican who has supported earlier overhauls of campaign-finance law , dismissed the Disclose Act as an election-year stunt by Democrats . `` I have long believed that Americans have the right to know who is contributing to political campaigns , and I share the serious concerns of so many who believe that outside , third party groups , should not be allowed to raise and spend unlimited amounts of money without revealing the names of donors , '' Collins said in a statement to CNN . `` That is why it is so frustrating that we find ourselves , once again , being asked to consider a partisan bill that would not fix the real problem . '' Meanwhile , Obama and Romney have benefited from the deluge of campaign cash pouring in from the `` super PACs '' spawned by the Citizens United decision . Pro-Obama super PAC raises $ 6.1 million in June . The pro-Romney Restore Our Future Fund has raised more than $ 61 million , according to OpenSecrets and spent $ 53 million of that . The Obama-allied Priorities USA Action has raised nearly $ 15 million and spent about $ 10 million , while during the GOP primaries , the super PAC that backed former House Speaker Newt Gingrich 's failed campaign raised nearly $ 24 million and spent all but a few hundred thousand . Pro-Romney super PAC brings in $ 20 million in June . All unions combined have put about $ 52 million into the 2012 elections so far , according to OpenSecrets . About $ 30 million of that has gone to Democrats , roughly $ 4 million to Republicans and the rest to independent groups . Rarely do big-dollar donors to either candidate rush forward to give their names and declare how much they 've contributed . `` Money is never going to run short in our political system , '' the Sunlight Foundation 's Rosenberg said . `` Where it does run short is ( what 's fair ) for the average voter and the average citizen ... if you are a voter and you 're being bombarded with negative ads and you do n't know whose behind them . '' Does big money spell big trouble in campaigns ? CNN 's Joe Johns and Ted Metzger contributed .
Democrat and Republican leaders are battling over disclosing big-dollar donors . The House Oversight Committee is investigating the matter. Voters are the ultimate losers in this fight over transparency , some politicos say .