With the usual GOP suspect — Arizona Sen. John McCain — on an apparent hiatus from the cause, the hunt is on for a new Republican campaign finance champion in the Senate, where a soon-to-be-introduced Democratic counterpunch to a recent Supreme Court decision faces its stiffest competition.

By early next week, House and Senate Democrats are expected to introduce a bill designed to blunt Citizens United v. Federal Election Commission, which tossed out most restrictions on corporate political ad buys. While language is still being finalized, the measure will more than likely expand disclosure requirements for corporations and nonprofit organizations looking to take advantage of the more liberal regulatory landscape.

But even before its debut, the legislation faces uncertain prospects without bipartisan support — or at the very least, one Republican vote to push Democrats to the 60-vote threshold to cut off debate in the Senate.

To date, no Republicans are publicly backing the bill, which is being sponsored by Democratic Congressional Campaign Committee Chairman Chris Van Hollen (Md.) and Senate Rules and Administration Chairman Charles Schumer (D-N.Y.), whose panel has jurisdiction over campaign finance matters.

The absence of McCain has been striking to colleagues and outside groups on both sides.

“Given that [McCain] made the centerpiece of his earlier political efforts governmental reform and campaign finance reform in particular, it’s curious,” said Marc Elias, a Democratic campaign finance lawyer at Perkins Coie. “It seems clear that he is not planning on being a leader on reform, but is he really willing to vote against reform?”

Elias and other observers say that the day of reckoning for possible McCain support is Aug. 24, when Arizona voters take to the polls to decide the 2008 GOP presidential nominee’s fate against his primary opponent, former Rep. J.D. Hayworth (R-Ariz.). Many observers speculate that after the primary, McCain may pivot toward the middle and could ultimately back a Schumer-Van Hollen bill.

“Perhaps he’s planning on voting for it, but because of political considerations or other things he just doesn’t want to be out front,” Elias said. “At the end of the day, it may very well be that he doesn’t want to be a proponent of it, but that’s different than saying he wouldn’t vote for it. … It may just be that he’s saving for the last minute that decision.”

Some campaign finance reformers have already written off any prospects of McCain support. Public Citizen lobbyist Craig Holman said McCain’s political aspirations are trumping his ideological zeal. The former moderate torchbearer, who recently shunned his “maverick” moniker to a reporter, faces highly energized GOP midterm voters — and less-than-certain prospects four months out.

“I have not been looking for Sen. McCain to support our causes since he started running for president,” Holman said. “McCain has pretty much the same problem going on right now as he did in his presidential campaign. He’s making every effort to appeal to the hard-core Republican base, and he has determined that it requires him to shift away from being a reform advocate for the time being.”

A McCain spokesman declined to comment for this article, but a Republican strategist agreed that the Senator’s silence on a Citizens United bill may end as he pivots toward November.

“It’s less the substance than it is being tagged in the primary as voting against the party,” the Republican strategist said. “It’s a hard decision for him because he doesn’t want to be seen as voting against the party.”

Polling suggests that the high court’s decision is unpopular with general-election voters, the source said, and McCain is merely staying quiet to avoid being associated with Democrats while he is in the midst of the primary fight.

Brad Smith, a former Republican FEC chairman, said the bill is doomed sans McCain.

“It’s damaging to the bill’s prospects,” Smith said. “I believe that Sen. McCain believes in this issue sincerely, but he’s not an idiot. … This truly is not bipartisan legislation. It’s a partisan effort to silence political opponents.”

In lieu of McCain’s pre-Labor Day support, sources said Democratic leaders are trying to sell the bill to Republican Maine Sens. Olympia Snowe and Susan Collins, both of whom voted for the McCain-sponsored Bipartisan Campaign Reform Act of 2002.

Campaign Legal Center spokesman David Vance wrote in an e-mail that “in terms of getting to 60 votes, theoretically only one Republican vote would be needed to break a filibuster.”

“There were 10 other Republicans in the Senate who joined Sen. McCain in voting for final passage of the Bipartisan Campaign Reform Act in 2002,” Vance wrote. “A number of those Senators are still in office.”