By Richard Cowan and Susan Cornwell

WASHINGTON (Reuters) - Defying President George W. Bush's veto threat, the House of Representatives on Wednesday approved a bill providing new war funds while setting a timeline for the withdrawal of all U.S. combat troops from Iraq by March 31 next year.

By a mostly partisan vote of 218-208, the Democratic-led House narrowly approved the $124-billion (61.9 billion pound) emergency spending bill, ignoring Bush's promise to veto any bill that sets deadlines for withdrawing U.S. troops from Iraq.

The Senate is expected to approve the legislation on Thursday, sending it to Bush for what would be only his second veto in more than six years as president.

"Tonight, the House of Representatives voted for failure in Iraq and the president will veto its bill," White House spokeswoman Dana Perino said.

Democratic Rep. John Murtha, the Pennsylvania Democrat who has led efforts to end the war, said it was "ironic" that Bush will be sent the bill on Tuesday, the fourth anniversary of the president's speech aboard a U.S. aircraft carrier emblazoned with a banner claiming "mission accomplished" in Iraq.

The House vote came hours after the U.S. commander in Iraq came to Capitol Hill to brief lawmakers on the status of the war, a briefing the White House hoped would bolster support.

But Democrats were not swayed.

"This bill gives the president the exit strategy from the Iraqi civil war that up until now he has not had," said House Appropriations Committee Chairman David Obey, a Wisconsin Democrat.