WASHINGTON -- Senate Democrats took the first step to try and stem the flood of unregulated dark money that's swamped politics since 2010, with a first procedural vote on a constitutional amendment to allow stronger regulation of campaign finances.

The unregulated cash spigot opened in 2010, when the Supreme Court's Citizens United decision reversed decades of precedent to allow corporations, unions and, ultimately, wealthy individuals to spend unlimited sums on political activity. The conservative majority on the high court opened the floodgates wider in April in its McCutcheon ruling by eliminating the overall limit on how much a single donor may give directly to all federal candidates and party committees.

"We are here today to overturn Citizens United," said Sen. Tom Udall (D-N.M.), a lead sponsor of the measure, who spoke at a press conference Monday unveiling a purported 3 million signatures backing the constitutional amendment.

"We are here to say as strongly and forcefully as we can, 'Corporations are not people.' And were are here to take back our democracy from billionaires," Udall said. "Our elections are not auctions up to the highest bidder."

In the 2014 election cycle, federal candidates and political parties have raised more than $1.8 billion. Independent super PACs, nonprofits and unions have spent an additional $190 million to support or oppose individual candidates. The entire campaign is expected to cost more than $4 billion -- a record for a midterm election.

Udall's proposed constitutional amendment would reverse not only the court's Citizens United ruling, but its 1976 decision in the Buckley v. Valeo case that underlies the nation's entire system of campaign finance. It would further allow states to determine whether corporations should be treated as legal persons under the law.

The Buckley decision came after Congress enacted campaign finance regulations in the 1970s to limit the amount donors could give and candidates could spend as a response to the scandals unearthed in the Watergate hearings. In its 5-4 decision, the court gutted the law by leaving limits on donor contributions intact, while striking down limits on the amounts candidates could spend.

The ruling is famous for applying First Amendment free speech protections to the spending of money -- a legal argument first made in 1971 as a response to the clamor for limitations on campaign spending. During arguments, Justice Potter Stewart spoke the lines that notoriously summarize the court's ruling: "Speech is money and money is speech."

But Democrats -- and in the past, many Republicans -- argue that the unlimited spending on campaigns that the Supreme Court has permitted over the last four years had rendered the free speech protection ridiculous.

Senate Majority Leader Harry Reid (D-Nev.) regularly takes to the Senate floor to rail specifically against the billionaire Koch brothers, saying their millions in spending overwhelms the free speech of ordinary people. Reid did so again Monday, declaring that the current state of affairs gives only the wealthy a loud voice in politics.

"The only people that would have a vote are these mega-billionaires who are trying to buy our country," said Reid. "They are trying to buy America, at every level of government. Why? Because they want to make more money."

Reid and other Democrats noted that bills that would at least require the disclosure of all campaign spending have been blocked by the GOP, leaving few other avenues for reform-minded lawmakers to pursue.

"We can keep the status quo, or we can change the system and restore the fundamental principle of one American, one vote," Reid said.

But Republicans contended that the bid to change the Constitution was really just an election-year gambit, and that if it succeeds, it would abridge Americans' right to speak, in this case by barring them from spending as much money as they want on politics.

"As written, the First Amendment does not permit regulation of the sort the majority wishes to impose, so they have decided to rewrite it," said Sen. Pat Roberts (R-Kan.), making the case for his side.

"This is incredible, and a sad demonstration of the lengths to which this majority is willing to go in its quest to retain power," Roberts said. "They want to grant themselves the power to silence their opposition."

Democrats countered that their measure would affect Democratic big-spenders just as much as the GOP's.

Asked at the news conference about the idea of trying to silence their opposition, Sen. Amy Klobuchar (D-Minn.) noted that such measures used to be bipartisan. "Dark money should not be allowed. You can't have it on one side and not the other," Klobuchar said. "I don't think we should have those kinds of independent expenditures, whether they're on the left or the right."

The Senate voted 79 to 18 Monday to proceed with debate on the measure, although Democrats expected Republicans would filibuster the measure at the next procedural vote, coming as soon as Wednesday.

The debate is leading to the fourth vote in the Senate's history on a constitutional amendment related to campaign finance. Former Sen. Fritz Hollings (D-S.C.) was the standard bearer for the amendment from 1987 until his retirement in 2004. Hollings, each time with bipartisan support, brought the amendment up for a vote in 1988, 1998 and finally in 2001.

The high-water mark for success came early for amendment supporters. In 1988, the amendment failed to pass a cloture vote and was blocked, even though it received a majority vote of 53-47. In both 1998 and 2001, Hollings' amendment failed to gain a majority of votes.

Udall has taken up Hollings' role as chief proponent of excising big money from the political system. The current amendment is endorsed by 46 Democrats and the two independents caucusing with the Democratic majority. The only Democratic senators not cosponsoring the amendment are Sens. Joe Donnelly (Ind.), Tim Kaine (Va.), Mary Landrieu (La.), Pat Leahy (Vt.), Mark Pryor (Ark.) and Mark Warner (Va.).

Behind the amendment is growing support for change in the campaign finance system. Since the Citizens United decision, 16 states and nearly 500 local governments have passed resolutions calling for a constitutional amendment to overturn it and the 1976 Buckley decision.