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J. Scott Applewhite, File, Associated Press
In this April 10, 2014 file photo, Rep. Gerald Connolly, D-Va. listens during a hearing on Capitol Hill in 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.

WASHINGTON — Congress' upheaval over trade has exposed lots of turmoil within the House Democratic caucus.

It's a caucus that's grown smaller and more liberal in recent years as moderates have lost their seats.

Those Democrats who remain must answer to ideologically driven voters and to labor unions fighting their own battles for survival.

The result is a caucus dominated by some of its most liberal members. The remaining centrists are left to wonder whether that'll make it harder for their party to regain the House majority anytime in the next decade.

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.