Updated 6:17 p.m.

A stream of amicus briefs to the Supreme Court on a closely watched political spending case turned to a flood by the end the week, as Members of Congress, attorneys general and advocacy groups weighed in.

The big surprise was a bipartisan brief from Sens. John McCain (R-Ariz.) and Sheldon Whitehouse (D-R.I.) urging justices to let stand a Montana law that bans corporate political expenditures. The court is weighing whether to take up a constitutional challenge to the law, and today marked the deadline for friend of the court briefs in that decision.

“Evidence from the 2010 and 2012 electoral cycles has demonstrated that so-called independent expenditures create a strong potential for corruption and the perception thereof,” the two Senators wrote in their brief. “The news confirms daily that existing campaign finance rules purporting to provide for ‘independence’ and ‘disclosure’ in fact provide neither.”

Four House Democrats also weighed in with a brief: Reps. Robert Brady (Pa.), Charlie Gonzalez (Texas), Zoe Lofgren (Calif.) and Chris Van Hollen (Md.), who has authored the House version of a campaign finance disclosure bill. The case is Western Tradition Partnership v. Montana.

“This case is a chance for the court to clean up some of the mess it made in Citizens United,” Brady, ranking member of the House Administration Committee, said in a statement, referring to the high court’s 2010 ruling to deregulate political spending. “Since that decision, we’ve seen the effect of unlimited, secret contributions, and it is nothing like the majority opinion envisioned.”

Briefs have also come into the court from the groups challenging the Montana law and their allies, including Sen. Mitch McConnell (R-Ky.).

“The ruling of the Montana Supreme Court is in direct contravention of this Court’s ruling in Citizens United,” McConnell wrote in a brief filed last month. “Nothing that has occurred since that ruling warrants its reconsideration. In fact, the central concerns expressed by those members of this court who dissented in Citizens United or joined earlier opinions sustaining campaign finance laws that limited speech have not been borne out by events of the past two years.”

The unusual number of amicus briefs reflect intense public interest in the Citizens United ruling and the unrestricted spending it has triggered, said Adam Skaggs, senior counsel at the Brennan Center for Justice at New York University’s School of Law.

“It’s pretty significant to have that many folks weighing in at this petition stage,” Skaggs said. “And I think it just underscores the significance of the Citizens United case, and the broad, widespread concern about that case for elections and democracy.”

Montana was one of several states whose prohibitions on corporate spending were technically nullified by the Citizens United ruling.

But state officials refused to take their law off the books on the grounds that the state has a special history of corporate campaign finance abuses, effectively inviting a legal challenge.

Three Montana corporations challenged the law on the grounds that it flies in the face of the Citizens United ruling. But the Montana Supreme Court in December rejected that challenge, throwing the ball to the Supreme Court. In February, justices stayed the Montana ruling so they could consider whether to take up the case.

In issuing the stay, Justice Ruth Bader Ginsburg stirred speculation that at least some justices believe the high court should revisit Citizens United. “A petition for certiorari will give the court an opportunity to consider whether, in light of the huge sums currently deployed to buy candidates’ allegiance, Citizens United should continue to hold sway,” Ginsburg wrote in a statement, joined by Justice Stephen Breyer.

The high court now faces three options. One, it could deny the cert petition outright, allowing the state Supreme Court ruling — and the Montana corporate spending ban — to stand. Few expect the Supreme Court to take that route, as it would effectively invite other states to pursue additional corporate spending bans at odds with Citizens United.

Two, the high court could summarily reverse the state Supreme Court’s ruling, as the Montana law’s challengers request.

Three, the Supreme Court could take the case up for argument, inviting another round of amicus briefs, and that could draw national attention on the scale of the constitutional challenge to President Barack Obama’s health care law. A decision is expected by the end of June.

In their amicus brief, McCain and Whitehouse argue that the high court should either deny cert and let the Montana corporate spending ban stand or take up the case for full review. Another amicus brief filed this week by the Brennan Center for Justice, and signed by several prominent constitutional scholars, makes a similar case.

The unrestricted super PACs that have emerged in the wake of the Citizens United ruling have spent close to $100 million in this election, the Brennan Center brief notes, much of it raised in donations of $1 million or more from wealthy individuals such as casino magnate Sheldon Adelson, who with his family gave $20 million to a super PAC backing former Speaker Newt Gingrich (R-Ga.).

Other groups that have filed briefs before the Supreme Court include Citizens United, the conservative group that prompted the court’s 2010 ruling to begin with and that sided with those challenging the Montana law, and the Campaign Legal Center, which was joined by more than a dozen civil rights, legal and watchdog organizations and sided with Montana.