WASHINGTON — In a shocking political development, Senate Minority Whip Trent Lott, R-Miss., will announce Monday that he'll resign by the end of the year, congressional officials confirmed.

Lott, who won re-election just a year ago, is in his fourth Senate term. His office issued an "urgent" news release this morning announcing two news conferences, at 11 a.m. CST at the LaFont Inn in Pascagoula, Miss., and at 3 p.m. at the Edison Walthall Hotel in Jackson, Miss.

Lott hasn't yet given a reason for his sudden exit, but speculation is centering on a possible academic posting. A Senate aide, who spoke on the condition of anonymity because he hadn't been authorized to speak publicly, said there was no known health reason.

"He talked about retiring before," said the staffer, noting Lott's decision to seek re-election only after Hurricane Katrina devastated Mississippi's coast in 2005.

Mississippi law lays out different scenarios on replacing a senator, meaning that the timing of Lott's retirement will determine what happens with his successor. If Lott retires in 2007, Gov. Haley Barbour will call a special election in 90 days. If Lott delays his retirement to 2008 — which is a year that a national election already is scheduled — the governor will appoint someone to the Senate until the election next Nov. 7.

The anticipated resignation roils Mississippi politics only days after Sen. Thad Cochran, R-Miss., decided to seek re-election. The opening of the seat doesn't immediately worry Republicans, said a top election aide who spoke on condition of anonymity. "Mississippi is a red state," the aide said.

Lott would be the sixth Republican senator to announce his retirement this year.

Among possible Republican contenders for his seat is Rep. Charles "Chip" Pickering, who announced his retirement from the House of Representatives earlier this year but has had his eye on a Senate seat for years.

Democrats who may weigh in include former Mississippi Attorney General Mike Moore.