Story highlights Sanders has refused to endorse Hillary Clinton

In meeting, Democrats booed Sanders after he didn't commit to a timeline for endorsing

Washington (CNN) House Democrats roughed up Sen. Bernie Sanders in a closed-door session Wednesday after he deflected questions about when he would formally back Hillary Clinton for president, with a group of members booing him at one point, according to three Democrats who attended the meeting.

The vast majority of House Democrats endorsed Clinton's candidacy early in the primary process, but Sanders was invited to attend the weekly meeting as a courtesy extended to all presidential candidates. When Clinton appeared last month, she was applauded and praised for her commitment to help Democrats regain control of Congress this fall.

Many Democrats have been reluctant to publicly criticize Sanders for continuing his campaign because they want to ensure that the supporters he activated through the long primary contest will come out and vote for the Democratic ticket in November. But some members let out their frustration with the Vermont senator Wednesday morning, with one member calling Sanders' appearance before the caucus a "total display of self-obsession."

During the discussion, Sanders told the group "the goal is not to win elections" and took a dramatic pause, before adding that the aim was to win the battle of ideas, according to multiple sources in the room.

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