Amid speculation that Senate Democrats will bring up a campaign finance disclosure bill as early as June, the League of Women Voters has launched a $90,000 radio ad campaign calling on four GOP Senators to “tell us you support full disclosure.”

The ads target Tennessee Sens. Lamar Alexander and Bob Corker and Maine Sens. Susan Collins and Olympia Snowe because they are “key actors on this important issue,” said the group’s president, Elisabeth MacNamara, in a statement.

The lawmakers “said supportive things about disclosure” in the past, said Kelly Ceballos, the league’s communications director. But Ceballos said the ads, which will run through June 1, are not specifically aimed at convincing the Senators to support legislation known as the DISCLOSE Act.

“It is about this issue,” Ceballos said. “But we certainly support the legislation.”

Still, the letter comes on the heels of a May 16 letter signed by 38 pro-reform organizations, including the league, urging all Senators to vote for the DISCLOSE Act. Supporters have no hope of winning approval for the legislation, which would require politically active groups to more fully report who’s backing the ads they run, without at least some GOP support.

A broader version of the DISCLOSE Act won approval in the House in the previous Congress but fell just short in the Senate, where not a single Republican backed the bill.

This year’s version, sponsored in the Senate by Sen. Sheldon Whitehouse (D-R.I.), is more narrowly tailored. Last week, the newspaper the Hill quoted erstwhile reform champion Sen. John McCain (R-Ariz.) saying that he had discussed the issue with Whitehouse and a couple of other Democrats.

Also last week, McCain teamed up with Whitehouse on an amicus brief to the Supreme Court that urged justices to uphold a Montana law that bans corporate political spending.

McCain, who authored the 2002 soft money ban with then-Sen. Russ Feingold (D-Wis.), dropped political money as a signature issue when he ran for president in 2008. But in recent months, McCain has publicly denounced the Supreme Court’s 2010 Citizens United v. Federal Election Commission ruling to deregulate corporate and union political spending.

McCain could not be reached for comment, but a spokesman said the Senator is “always interested in looking for ways to address the issues that Citizens United has unleashed.” It’s too early to say whether McCain would support the DISCLOSE Act, his spokesman added, noting that the Senator has “concerns” about Democratic proposals that might impose different rules on politically active corporations, unions and individuals.

“We’re not counting out the possibility of getting some Republican votes,” said one reform advocate working with Senate Democrats on the issue. “No one expects this to be enacted this year or to break the filibuster. But we can make important progress. We can get it to the floor, we can have a national debate, we can get people on the record and we can take it into the election.”

Senate Democrats backing the DISCLOSE Act, including Whitehouse and five others, took to the Senate floor May 23 to tout the bill and to decry the growing role of large, secret campaign contributions in the wake of Citizens United.

“All of our predictions in the aftermath of the flawed Citizens United decision, unfortunately, are coming true,” said Sen. Charles Schumer (D-N.Y.) on the floor. “This decision handed a megaphone to the wealthiest voices among us and strapped a muzzle on every other American.”

The league would like to take its pro-disclosure message “to the whole country” but as yet does not have the resources for that, Ceballos said. The Maine and Tennessee radio ads were funded by an unusual grant from the Unitarian Universalist Congregation in Shelter Rock, N.Y.

Attempts to convince Republicans to rally behind the DISCLOSE Act will face stiff opposition from Senate Minority Leader Mitch McConnell (R-Ky.), a staunch opponent of campaign finance restrictions.

Alexander endorsed political money disclosure when he sought the GOP presidential nomination in 1999 but more recently faulted Democrats for trying to change the rules mid-election to discourage contributions to their opponents. The real solution is to take “all the limits off campaign contributions,” Alexander said at a March Senate Rules Committee hearing on campaign financing.