Senate Minority Whip Harry Reid (Nev.) will announce today he will run for Minority Leader to replace Sen. Tom Daschle (S.D.), who was defeated in his bid for a fourth term in an election that saw Republicans make significant gains in the chamber.

Daschle’s loss will shake up the Senate Democratic leadership structure, which has been largely unchanged since Reid was elected by colleagues to serve as Whip following his nail-biting 428-vote win in 1998. There is speculation Sen. Chris Dodd (Conn.) is also seriously considering seeking the top leadership post, a job he lost to Daschle by one vote following Congressional Republicans’ 1994 election sweep.

Reid made his decision to seek the senior Democratic position Wednesday morning after Daschle called to tell him that he was conceding the race to former Rep. John Thune (R-S.D.), the Nevadan’s senior aide said.

“Senator Reid will run for leader,” Reid’s chief of staff, Susan McCue, confirmed when reached in Nevada at 4:45 a.m. Eastern Standard Time.

Reid is planning to make the official announcement today at a news conference in Nevada. Dodd’s spokesman could not be reached for comment early this morning.

Another person some Democrats might turn to for Daschle’s post is Sen. Hillary Rodham Clinton (N.Y.), who is rumored to be eyeing a 2008 run for the White House.

Reid’s decision to run for Democratic leader will spark another leadership contest to succeed him as Democratic Whip. Sens. Byron Dorgan (N.D.) and Dick Durbin (Ill.) are said to both be interested in the Whip position, but others might also enter this open leadership race. Dorgan serves as the chairman of the Democratic Policy Committee, and Durbin is the Assistant Floor Leader.

Reid and Dodd each took steps to run for Democratic leader in December 2002 when Daschle seriously considered running for the Democratic presidential nomination and weighed whether to step down from his post. Daschle would later decide to forgo a run for the White House in order to seek re-election to the Senate. Daschle lost to Thune by a 2-point margin, 51 percent to 49 percent, in the most watched Senate race of the 2004 election cycle.

Dodd, Reid and Dorgan all won their re-election contests Tuesday by comfortable margins. Durbin easily won re-election two years ago.

In 2002, Reid’s office declared that 35 Democratic Senators had pledged to vote for him to be the next Democratic leader if Daschle stepped down. At the time, Dodd’s spokesman said the Connecticut Democrat had been approached by some colleagues urging him to consider the race if Daschle vacated the post.