Senior Democrats on and off Capitol Hill were becoming increasingly convinced late Tuesday that Rep. Rahm Emanuel (D-Ill.) is preparing to quit his post as the fourth-ranking House Democrat to accept a job as White House chief of staff in an Obama administration.

The move would mean House Democrats are losing one of their pre-eminent political strategists — and a figure many regarded as the Speaker-in-waiting — just as they gear up to unleash an ambitious agenda with what they expect will be expanded majorities in both chambers and one of their own in the White House.

“I think it’s a big loss for the House, and it would better if he stayed,” said Steve Elmendorf, a top Democratic lobbyist. “But being Speaker is theoretical, and this is something that’s right in front of him that he can have right now.”

Several sources said Emanuel has not yet informed his fellow House Democratic leaders of any decision, but that he has told other confidants that he is going to accept an offer to join Obama’s administration. This would be the second White House tour for Emanuel, who served as a top adviser to former President Bill Clinton.

Rumors of the move first surfaced in an Associated Press report late last week and were dismissed out of hand by Emanuel’s camp. Most Democrats assumed then that both the Obama campaign and Emanuel were seeking to avoid any appearance of drape-measuring before the Illinois Senator actually wrapped up a victory in the 2008 presidential race.

And Tuesday night, Emanuel’s staff continued to deny the suggestion he was in talks with an not-yet-elected Obama.

“Anyone familiar with Rahm Emanuel’s thinking knows he is solely focused on Election Day: a victory for Barack Obama, increasing the Democratic majority in the House, and bringing change to Washington,” said spokeswoman Sarah Feinberg.

Obama has relied on Emanuel’s counsel as the race wore on — a fact brought into relief in the immediate aftermath of the House’s failure to pass the Wall Street bailout package on its first try. On Sept. 29, with the House erupting in chaos after the measure fell 12 votes shy, House Democratic leaders huddled in a conference room in Speaker Nancy Pelosi’s (D-Calif.) suite of offices to discuss what to do next. Minutes into that session, Emanuel got a call on his cell phone from Obama — the Senator’s first to anyone in leadership after the vote, sources with knowledge of the meeting said.

Democratic sources said Obama would do well to have a hard-nosed politician with an insider’s grasp of Washington power like Emanuel at his side as he takes the reins at 1600 Pennsylvania Ave.

“If it’s true, it makes sense,” one said. “Rahm is very smart, knows how the White House and Congress work and would be a good link between the two.”