By Susan Cornwell

WASHINGTON, June 29 (Reuters) - U.S. Democratic leaders vowed Friday to keep up the pressure on President George W. Bush to end the Iraq war with more votes next month on withdrawing U.S. troops.

House of Representatives Speaker Nancy Pelosi said she would seek a vote on legislation instructing the Pentagon to begin pulling out U.S. troops within 120 days, similar to a provision Bush vetoed in the first version of a war spending bill Congress sent to him in May.

A comparable provision also is expected to come to the floor of the Senate in July. So far this year, Democrats there have been unable to get the 60 votes needed to overcome procedural hurdles to proposals to start a pullout from Iraq.

"I don't know if we can or not," said Senate Majority Leader Harry Reid said of clearing the 60-vote hurdle. "We're going to keep pushing, because it's the right thing to do."

At a news conference with Reid, Pelosi added: "The Republicans have the 60-vote barrier. The president has the (veto) pen. But we have the support of the American people, who want this war to come to an end."

The California Democrat said there were signs Republican attitudes toward the war were changing. In the Senate this week, a longtime White House supporter, Indiana Republican Richard Lugar, said Bush's policy was not working and time was running out to change course.

Reid, a Nevada Democrat, said that even with recent Republican shifts, the 60 Senate votes could be elusive but he felt he had to try.

"I feel that I -- and just not as a senator, but as a person -- have a moral obligation to do what I can to stop the death and suffering. And I think by pushing a withdrawal date, it does just that," Reid said.

Senate Armed Services Committee Chairman Carl Levin, a Michigan Democrat, is expected to introduce a troop withdrawal amendment to defense policy legislation the second week of July.

Other options for amendments on the Senate legislation, which are still being reviewed, could include rewriting Congress' 2002 war authorization, setting readiness standards for military units and cutting off all combat funds by April.

Senate Minority Leader Mitch McConnell, a Kentucky Republican, argues that lawmakers should wait until September, when the top U.S. commander in Iraq will make a progress report, before pushing for change in Iraq policy.

"The handwriting is on the wall that we are going in a different direction in the fall, and I expect the president to lead it," McConnell said last month.

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