U.S. Senator Bernie Sanders (D-VT), right, addresses a news conference in support of a proposed constitutional amendment for campaign finance reform, on Capitol Hill in Washington, D.C. (Jonathan Ernst/Reuters)

The debate about the pull that wealthy donors have in politics is taking center stage on the Senate floor this week as Democrats push a constitutional amendment that would give lawmakers more leverage to restrict campaign financing and spending.

The long-shot measure — dubbed by supporters the Democracy for All Amendment — is an attempt to provide elected officials with the legal authority to curb the big money that has buoyed super PACs and politically active tax-exempt groups since the Supreme Court’s 2010 Citizens United decision.

The bill cleared a procedural hurdle Monday that will allow for debate to begin. Debate is expected to continue until Thursday, when aides in both parties predict the measure will fall short on a party-line vote.

The bill, sponsored by Sen. Tom Udall (D-N.M.), says that Congress and the states can “set reasonable limits on the raising and spending of money by candidates and others to influence elections.” It also gives federal and state authorities the power to prohibit election spending by corporations, which was permitted by Citizens United.

Udall warned that “we are setting the stage for scandals” by not controlling campaign spending. Absent legislative action, “we’re headed back to the pre-Watergate era,” he said later.

The amendment faces steep odds: It would require two-thirds majorities in the House and Senate and then have to be ratified by three-quarters of the states.

“There is no doubt in my mind that that would never, ever, ever happen,” Sen. John Cornyn (R-Tex.) predicted Monday.

But the measure is being seized upon by both the left and right as a way to engage in a proxy fight over the role of money in politics — a major theme of this year’s midterm elections, in which billionaire donors such as industrialists Charles and David Koch and hedge fund manager Tom Steyer have played starring roles in campaign ads. Already, outside groups have reported spending nearly $200 million on this year’s races, according to the Center for Responsive Politics, a nonpartisan research organization. That’s more than triple this point in the 2010 midterms.

Both sides are seeking to make the most of the Senate vote. Senate Majority Leader Harry M. Reid (D-Nev.), who regularly uses Senate floor speeches to blast the growing influence of the Koch brothers, did so again Monday, suggesting that thanks to lax campaign finance laws, “families can’t compete with billionaires.”

If Congress doesn’t act to reverse the Supreme Court decision, “the only people that would have a vote are these megabillionaires who are trying to buy our country,” he said. “They are trying to buy America, at every level of government. Why? Because they want to make more money.”

Sen. Bernard Sanders (I-Vt.), a self-described socialist who caucuses with Democrats, dubbed the overhauling of the nation’s campaign finance system “the most important issue” facing the country. Failing to act to reverse the Supreme Court decision would risk moving the country “into an oligarchic form of government where a billionaire class can buy elections,” he said in an interview Monday.

Meanwhile, Republican senators are expected to use floor debate over the bill to spotlight the role of wealthy liberal donors and to suggest that Democrats are seeking to make political gains with less than two months until elections.

Sen. Pat Roberts (R-Kan.) called the measure “a sad demonstration of the lengths this majority is willing to go in its quest to retain power.”

Cornyn said that during the August recess, “not a single time did my constituents say we want you to go back to Washington, D.C., and vote to gut the First Amendment.”

But Democrats pushed back against suggestions that new limits on campaign spending would stifle free-speech rights.

“I didn’t see the word ‘cash’ in the First Amendment,” said Sen. Richard J. Durbin (D-Ill.), who predicted that the vote would break along party lines.

The bill has 48 co-sponsors — none of them Republican. Senate Minority Leader Mitch McConnell (R-Ky.) attacked it as an assault on free-speech protections. The goal of the Democrats, he wrote in a piece for Politico , “is to shut down the voices of their critics at a moment when they fear the loss of their fragile Senate majority.”

The absence of GOP support for the bill underscores a substantial shift within the party on campaign finance issues over the past several decades. Earlier versions of the constitutional amendment — which has been introduced in various forms since 1986 — have been co-sponsored by Republicans such as Sen. Ted Stevens of Alaska, Sen. Arlen Specter of Pennsylvania, Sen. Thad Cochran of Mississippi and Sen. John McCain of Arizona.