Sen. Bernie Sanders (I-Vt.) during a press conference at the U.S. Capitol in July. (Win McNamee/Getty Images)

This post has been updated.

Sitting in his Capitol Hill office this week, Sen. Bernie Sanders (I-Vt.) sighed deeply as he conceded that an issue near to his heart was on the verge of defeat.

With elections fast approaching and a fresh debate over military action underway, the U.S. Senate devoted most of its floor time this week to a longshot attempt to pass a constitutional amendment that would give lawmakers more leverage to restrict campaign financing and spending.

The Democracy for All Amendment, as supporters dubbed it, was an attempt to provide elected officials with the legal authority to curb the big campaign money that has buoyed super PACs and politically active tax-exempt groups since the Supreme Court’s 2010 Citizens United decision. Senate Democrats devoted time to the issue this week in hopes of building support among liberals and Democratic base voters concerned about the growing influence of outside political groups led by wealthier individuals, including industrialists Charles and David Koch and hedge fund manager Tom Steyer.

But the proposed amendment failed to survive a procedural vote Thursday afternoon. Senators agreed to begin debating the measure Monday night, but GOP senators united Thursday to block further consideration of the bill.

In an interview Wednesday, Sanders admitted that the amendment was likely to fail. But he said Congress and a grassroots movement of concerned Americans need to continue working to overturn the Citizens United ruling, which he considers "one of the worst in American history."

"The most important domestic issue facing the country is turning over Citizens United," Sanders said, because the court ruling "creates an open door … to pour unlimited sums of money into the political process."

"It’s a real undermining of American democracy and will undermine every aspect of our lives, because the candidates they support will help the rich and powerful and will ignore the needs of the middle class and working families," he added later.

Sanders was especially strident in his criticism of the Koch brothers, who -- along with other conservative donors -- back a network of tax-exempt groups that have run an estimated 44,000 election-related ads this year.

Failing to pass the constitutional amendment this week "means that the average American has one vote and the Koch Brothers have one vote plus the ability to put tens and tens of thousands of ads on the air. That is not democracy," Sanders said.

Sanders, 73, is a former mayor of Burlington, Vt. who later served in the House and was elected to the Senate in 2007. He has won each of his elections as an independent, and caucuses with Senate Democrats. His seniority has earned him the chairmanship of the Veterans Affairs Committee -- which gave him a prominent role in this summer's debate over the future of veterans' health-care.

The senator said he's still enjoying his work, but that he's working in a "very frustrating" environment.

Which might help explain why he's traveling to Wisconsin and Iowa this weekend.

In Wisconsin, he's scheduled to attend the "Fighting Bob Fest," a gathering of progressives. In Iowa, he's making stops in three cities. It's his second trip to the Hawkeye State this year -- and no national politician visits the early presidential primary state by accident in an even-numbered year.

"What I’m trying to figure out is, how flat can this state really be?" Sanders quipped when asked about his intentions in Iowa.

Turning serious, Sanders said the trip is part of his ongoing review of whether to run for president in 2016. And he admitted that he's considering whether to run either as an independent candidate or possibly as a Democrat.

If he runs, "I’m not going to run on gossip and I’ve never run a negative ad in my life. I’m not about to be sitting here attacking my opponents and hire 14 consultants to figure out what tie I wear," he said. "I will be running on solid issues, which talk about the collapse of the middle class, the need to develop policies that protect working people, dealing with income and wealth inequality, national health-care and all that stuff."

And yes, he said, the failure of the campaign finance amendment is likely to be a subject of conversation this weekend.

"My hunch is that the Koch Brothers would not pour hundreds of millions of dollars into my campaign," he said.

In response, Mark Holden, senior vice president and general counsel for Koch Industries, pointed to Democracy Alliance, a private group of wealthy donors who back liberal organizations.

“We think it is regrettable that with all the important issues facing our great country, Senator Sanders and many Democrat Senators wasted taxpayer money and time on this," Holden said in a statement. "We believe the Udall Amendment – which represents the first time in history that anyone has tried to amend the Bill of Rights – was contrary to our Constitution and the First Amendment.

"The Democracy Alliance and its allied organizations, many of whom support Senator Sanders and the Democrats who voted for this amendment, are engaged in the same activities they attribute to Koch," he said. "While we support their right to do this, their blatant hypocrisy is disappointing."

 Matea Gold contributed to this report.