WASHINGTON - Congress' upheaval over trade has exposed turmoil within a House Democratic caucus that's grown smaller and more liberal in recent years as moderates have been ousted in successive election bloodlettings.

Those who remain must answer to ideologically driven voters and labor unions fighting their own battles for survival, even if it means sidelining their own leaders and humbling their president in the process.

The result is a minority caucus dominated by some of its most liberal members, leaving the few remaining centrists to question whether that will make it harder for their party to retake the seats they need to regain the House majority anytime in the next decade.

Just as the tea party wing of the Republican Party has pulled the entire GOP to the right and hampered attempts at compromise on Capitol Hill, some now fear a similar dynamic is taking shape on the left.

"The real question is, are we going to try to broaden our caucus," said Rep. Gerry Connolly of Virginia, who unlike most Democrats is an outspoken supporter of President Barack Obama's trade agenda. "That means listening more to some of these swing districts and suburban districts which have a different economic outlook."

House Democrats celebrated earlier this month after they derailed Obama's bid for expedited trade negotiating authority by voting down a linked worker retraining program they long had supported. But now it looks like their victory may have been fleeting. Obama maneuvered with congressional Republicans to get the trade package back on track, clearing a key House hurdle Thursday and setting up make-or-break votes in the Senate in coming days.

The revival of the trade package inflamed labor unions and liberal groups that had fought ferociously to block it, including by running ads against otherwise friendly House Democrats and threatening to mount primary campaigns against them. Unions say past trade deals bled American jobs and tanked wages. They argue that granting Obama the power to finalize trade deals that Congress can accept or reject, but not amend, would lead to more of the same, including the 12-nation Trans-Pacific Partnership the White House has worked on for years.

"Democrats who allowed the passage of fast-track authority for the job-killing TPP, should know that we will not lift a finger or raise a penny to protect you when you're attacked in 2016, we will encourage our progressive allies to join us in leaving you to rot, and we will actively search for opportunities to primary you with a real Democrat," Jim Dean, head of Democracy for America, said in a statement following Thursday's House vote.

It's the kind of vicious infighting that has characterized GOP politics since the tea party rose in 2010 and began trying to oust anyone who disagreed with its conservative tenets. Few believe that the fissures within the Democratic Party are as stark, noting that trade is an unusually divisive issue for the party. Yet leaders are openly alarmed at the internal conflict and are warning that Democrats must move on quickly to more harmonious topics - or possibly face even more election losses.

"We risk marginalizing ourselves if we dominate our discourse with divisions and process arguments," said Rep. Steve Israel of New York, a member of the House Democratic leadership team. "At the end of the day we have the responsibility to find common ground that's going to allow us to win back this majority."

Democrats lost their majority in 2010 and now number 188 in the House, compared with 246 Republicans, with one seat vacant. Many more lawmakers in both parties represent safe seats, and there are fewer swing seats nationally than in years past, said Matt Bennett of the centrist Democratic group Third Way.

For Democrats, "the caucus has become smaller in every way, both physically and ideologically," Bennett said.

House Democrats also rely on an aging generation of leaders and have not seen new blood at the very top in years.

House Minority Leader Nancy Pelosi, D-Calif., who is 75, has led the caucus since 2002, into the majority and out again. She has been known for ruling with an iron fist, yet her unpredictable moves on the trade package raised eyebrows around the Capitol. She announced at the last minute she would side with her caucus' liberals and vote against the legislation even after working with Republican leaders toward compromise and conferring privately with Obama.

Still, Pelosi's position is secure, and many Democrats dispute the suggestion that their caucus has grown more ideologically uniform - or they simply argue that it's no cause for alarm.

"People who pretended to be Democrats but actually were voting with the Republicans got wiped out because the voters decided they'd rather have real Republicans in those seats," said liberal Rep. Alan Grayson of Florida. "What it's allowed us I think is to show the unity that the people who are relying upon us deserve, rather than have people working against Democratic principles from within."