A day after 43 Senate Republicans unanimously voted to block the DISCLOSE Act from receiving an up-or-down vote, Democrats will try again for cloture at 3pm on Tuesday.

The measure, which lawmakers drafted in response to the Supreme Court’s 2010 Citizens United ruling, “would require independent groups to disclose the names of contributors who give more than $10,000 to independent groups for use in political campaigns.” Here is what you should know about the measure:

1) It’s all about disclosure. The bill’s sole purpose would be to require outside groups who can currently spend unlimited sums of money on “independent expenditure” ads attacking and supporting presidential and other candidates to identify who is paying for the ads. Under current law, a 501(c)(4) tax-exempt group like Karl Rove’s Crossroads GPS can spend millions of dollars on attack ads without citizens ever knowing who is paying for them. Under the DISCLOSE Act, if a group spends more than $10,000 on political ads in an election cycle, it would have to identify the donors funding the efforts.

2) The Supreme Court backed disclosure. In his 5-4 majority opinion in the Citzens United v. FEC case, Justice Anthony Kennedy wrote “disclosure is a less restrictive alternative to more comprehensive regulations of speech.” By an 8-1 majority, every Justice but Clarence Thomas agreed that Congress had acted properly when it required that donors be identified for political ads that do not expressly advocate for or against a candidate. But, while these indirect ads come with disclosure, Congress did not anticipate that Citizens United would allow outside groups’s ads to directly tell voters to support or oppose candidates, leaving a major loophole.

3) Republicans used to support disclosure. When Congress considered the McCain-Feingold campaign finance reform law in the early 2000s, opponents consistently argued that complete disclosure, rather than regulation, was the best campaign finance law. Senate Republican Leader Mitch McConnell (R-KY), who now dismisses disclosure as government-supported “harassment and intimidation,” once endorsed the concept and asked “why would a little disclosure be better than a lot of disclosure?” Fourteen current Senate Republicans who now oppose the DISCLOSE Act voted in 2000 for similar disclosure for 527 committees, the forerunner to these 501(c)(4) outside spending groups. And even Sen. John McCain (R-AZ), who this March warned the lack of disclosure for independent spending would lead to “huge scandals” still joined with the Republicans to stop the bill in 2010 and yesterday.