WASHINGTON — Opponents of Citizens United, the controversial Supreme Court decision that gave corporations and special interest groups the right to contribute unlimited funds to political campaigns, on Tuesday renewed their calls for comprehensive campaign finance reform.

At a hearing before the Senate Judiciary subcommittee on the Constitution, civil rights and human rights, leading Democratic members of Congress argued a constitutional amendment was necessary to overturn the measure.

“Congress owes it to the American people to fully study, discuss and debate the merits of a constitution amendment,” said Sen. Max Baucus, a Democrat from Montana. “It’s clear that action is needed to restore Americans’ faith in our political and electoral process.”

Joining Baucus were Sens. Mark Udall, Democrat of New Mexico, and Bernie Sanders, Independent of Vermont, as well as Rep. Donna Edwards, Democrat of Maryland. They argued that amending the Constitution is the most efficient solution.

“This is poisoning our democracy and often we don’t even know who is doing the poisoning,” said Udall, adding Republican support is needed to successfully enact comprehensive reform.

Last month, the Supreme Court struck down a ban by Montana to limit political campaign spending. In a 5-4 ruling, the high court ruled the Citizens United law applies to state and local elections.

Witnesses called to testify at Tuesday’s hearing answered questions about the impact of Citizens United on local and national elections and offered suggestions on ways to improve the current system.

“The solution is rather obvious,” said Ilya Shapiro, senior fellow at the libertarian think-tank CATO Institute in his prepared testimony. “Liberalize rather than further restrict the campaign finance regime. Get rid of limits on contributions to candidates – by individual, not corporations – and then have disclosures for those who donate some amount big enough.”

The panel also discussed the DISCLOSE Act, a bill introduced in the House and Senate earlier this year that would require individuals to divulge political contributions.

“However important disclosure is, disclosure alone could not reveal the actual influence of unlimited independent expenditures,” said Laurence Lessig, professor of law at Harvard Law School. “Disclosure may be essential, but disclosure is not enough.”

The hearing comes days after the deadline for second-quarter corporate lobbying and Super PAC campaign expenditures. Priorities USA Action, which supports President Barack Obama, received $6.1 million in June and Restore our Future Super PAC, which backs GOP challenger Mitt Romney, raked in $20.7 million, according to the Center for Responsive Politics.