Centrist Democratic think tank Third Way came under fire from some liberals in Congress this week after the group published an op-ed in the Wall Street Journal attacking the economic populism of Sen. Elizabeth Warren (D-Mass.). (Alex Wong/Getty Images)

An ugly feud broke out among Democrats this week after a centrist Democratic think tank criticized Sen. Elizabeth Warren’s populist policies, drawing an angry response from liberal groups that support the Massachusetts lawmaker.

The battle began with an op-ed in the Wall Street Journal by leaders of Third Way, who noted the passion that some progressives feel for the economic populism of Warren and Bill de Blasio, New York’s mayor-elect. “Nothing would be more disastrous for Democrats,” they wrote. They got tougher after that, using words such as “reckless” and “fantasy” to describe Warren’s recent proposal to increase Social Security benefits.

On Thursday, under pressure from liberal groups that consider Warren their champion, several members of Congress who serve as honorary co-chairmen of Third Way entered the fray.

“I’m tired of the politics of polarization,” Rep. Ron Kind (D-Wis.) said in a statement. “We need to find ways to build bridges, not tear them down. Opinion pieces attacking people are unproductive, and do nothing to help solve the challenges that our nation is facing.”

Kind and others stopped short of breaking their ties with Third Way, as several groups, including MoveOn.org and the Progressive Change Campaign Committee, had called on them to do. But their rapid responses highlighted the growing influence of liberals within the Democratic Party, including some who are keen for an alternative to Hillary Rodham Clinton and have encouraged Warren to run for president in 2016. The freshman senator said this week that she plans to serve out her term and won’t run for the White House.

Several of Warren’s defenders complained about what they said were Third Way’s close ties to the financial world, and Warren, the driving force behind the creation of the Consumer Financial Protection Bureau, fired her own shot in that direction. In a letter sent Wednesday, she asked the chiefs of the country’s six largest financial institutions to voluntarily disclose their contributions to think tanks.

Third Way showed no sign of stepping away from the fight. The op-ed, published Monday, was featured prominently on the group’s Web site Thursday.

“We have a very significant disagreement that is about policy,” said Jonathan Cowan, president of Third Way, who wrote the piece with Jim Kessler, the group’s policy chief. “It’s a huge argument, and it’s in­cred­ibly important and healthy to have a big-tent Democratic Party that can vigorously debate big policy disagreements,” Cowan said.

Rep. James E. Clyburn (S.C.), the assistant Democratic leader and another Third Way co-chairman who was encouraged to break with the think tank, had no comment Thursday, a spokesman said.

A spokesman for Rep. Joseph Crowley (D-N.Y.), another co-chairman, said the congressman “has worked with Third Way on a range of issues, such as immigration reform and the Affordable Care Act, but on this matter they strongly disagree. The congressman has and will continue to fight to protect Social Security and Medicare.”

Rep. Allyson Y. Schwartz (D-Pa.) said through a spokesman Wednesday that the op-ed was “outrageous.” Rep. Jared Polis (D-Colo.) was more cautious. “I like Elizabeth Warren. I like Third Way. I hope they can learn to get along better,” he said in a statement.