A leading liberal Democrat and chairman of the powerful U.S. House of Representatives' Appropriations Committee, intends to retire at the end of his term this year, Democratic sources said Wednesday. It is another blow to Democrats defending their majority in an election season of voter discontent.

Rep. David Obey, 71, who has served in the House for more than 40 years, faced a potentially bruising re-election campaign ahead of the November elections. Democrats now control both chambers of Congress and the White House but are at risk of seeing their majorities dwindle or disappear.

In years in which there is no presidential voting, the party that controls the White House historically has lost seats. The Obama administration is battling to keep any losses to a minimum.

Obey's office issued a statement saying he would make a major announcement later but disclosed no details. The Democratic sources spoke on condition of anonymity to speak frankly about Obey's decision before the announcement.

Obey, among a handful of veteran House Democrats who had been bracing for competitive races later this year, has routinely won re-election easily despite representing a competitive district in the Midwest state of Wisconsin. He won in 2008 with 61 percent of the vote. But he has never faced the level of competition as he does this year as voters sour on Washington.

Sean Duffy, 38, a Republican district attorney, is seen as the favored candidate in his party's primary, and his candidacy has attracted the backing of Republicans in Washington as well as the party's 2008 vice president nominee, Sarah Palin, and tea party activists, so named for a U.S. protest against British taxes in early U.S. history.

Obey came to the House during the tumult of the Vietnam War, when it was dominated by Southern conservative Democrats. He earned a reputation as a reformer over the years and is a longtime ally and confidant of House leader, Speaker Nancy Pelosi, a Democrat.

He also can have a gruff, sometimes prickly demeanor and does not suffer fools gladly.

He first became chairman of the Appropriations Committee in 1994, and was a top architect of President Barack Obama's economic stimulus bill.

Republicans took a measure of credit for forcing Obey's retirement.

"There is no question that David Obey was facing the race of his life and that is why it is understandable that the architect of President Obama's failed stimulus plan has decided to call it quits," said Ken Spain, a spokesman for the Republican campaign committee.

Democratic officials declined to respond, saying they would wait for Obey's formal announcement.