WASHINGTON — Several leading congressional Demo-crats complained Thursday that President Barack Obama’s troop withdrawal plan would leave behind too many U.S. forces in Iraq, exposing a fault line between the White House and its Capitol Hill allies.

As Obama prepared to fly to Camp Lejeune, N.C., today to announce his decision, legislative leaders signaled skepticism about the prospect of keeping a residual force of as many as 50,000 troops in Iraq beyond summer 2010. The leftover units would focus on training Iraqi security forces, hunting down terrorists and protecting U.S. installations.

“I have been one for a long time who has called for significant cutbacks in Iraq,” Sen. Harry Reid, a Nevada Demo-crat and the Senate majority leader, told reporters before heading to the White House for a briefing. “I’m happy to listen to the secretary of defense and the president, but when they talk about 50,000, that’s a little higher number than I anticipated.”

Obama consistently said during the presidential campaign that he would leave some forces even after he withdrew all combat brigades. The plan he intends to unveil today will withdraw at least 90,000 of the 142,000 troops in Iraq by August 2010. That would be about 19 months from his inauguration, or three months longer than the time frame he outlined as a candidate. Sensitive to criticism, the administration plans to call the remaining troops a “transition force” rather than a “residual” one.

The White House declined to respond directly to Democratic criticisms before Obama’s speech.

“The president asked his — the national security team — to put together a plan that they and he believed would accomplish the goal of removing our combat forces from Iraq in the most responsible way,” said Robert Gibbs, the White House press secretary.

Sen. Jack Reed, a Rhode Island Democrat who accompanied Obama to Iraq last summer, said a remainder force of 50,000 was appropriate.

“It seems logical to me you would need a force of around that level,” he said in an interview. “The critical issue here is the missions that these troops are going to perform.”

Democratic criticism began Wednesday when Rep. Nancy Pelosi, of California, the House speaker, said she did not understand “the justification” for 50,000 troops. Others echoed her language on Thursday.