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WASHINGTON, D.C. (CBSMiami/AP) — Bernie Sanders is facing intense pressure from House Democrats to endorse Hillary Clinton’s presidential campaign as rank-and-file lawmakers confronted him Wednesday behind closed doors with shouts of “Timeline! Timeline!” and even scattered boos.

Sanders, arriving with House Democratic leader Nancy Pelosi, was pressed directly by Democrats irritated by his slow-moving support for their presumptive presidential nominee.

Yet to the frustration of his listeners, the Vermont senator never answered their question, refusing to give a timeline for when he would get behind Clinton.

At one point he said, “Our goal is not to win elections,” and then paused. Democrats booed until Sanders completed his thought by saying, “but to transform America” in order to win elections.

The heated exchange came just three weeks before the start of the Democratic convention in Philadelphia. It happened during House Democrats’ regular weekly caucus meeting, to which Sanders had been invited some time ago. He appeared at a moment when many Democrats have run out of patience with Sanders, who is holding out for various items in the party platform.

A Democrat who attended the session provided details, demanding anonymity in order to speak freely.

“He was pressed. There was friendly heat,” said Rep. Gerald Connolly, D-Va.

Rep. Sander Levin, D-Mich., said there was a “general feeling that it is important that he endorse right away,” adding that it was expressed directly to Sanders. “In terms of the timing of endorsement it wasn’t clear.”

Sanders has said he will vote for Clinton but has not yet offered a full-throated endorsement, even as Democratic leaders like President Barack Obama and Massachusetts Sen. Elizabeth Warren have swung firmly behind the former secretary of state and appeared by her side at high-profile rallies. Sanders has been pushing for changes to the party platform that would specify the need for a $15 an hour federal minimum wage, opposition to the Trans-Pacific Partnership and more.

Most of the House Democrats have supported Clinton’s campaign throughout.

Rep. Joyce Beatty, D-Ohio, told Sanders that she felt “like a hostage” given his multiple demands for the party’s platform that will be considered at the convention, according to the Democrat who demanded anonymity.

Sanders instead pointed to progress with Clinton, telling reporters after the meeting that he applauded a “very bold initiative” by the former secretary of state to make college more affordable. But declined to say when he might endorse Clinton.

“We are working with Secretary Clinton on some initiatives. I hope we can reach agreement on them sooner rather than later,” he said.

(TM and © Copyright 2016 CBS Radio Inc. and its relevant subsidiaries. CBS RADIO and EYE Logo TM and Copyright 2016 CBS Broadcasting Inc. Used under license. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed. The Associated Press contributed to this report.)