In a surprise development, Rep. Mark Udall (D-Colo.) announced late this afternoon that he would forgo the 2006 governor’s race in the Centennial State and seek re-election instead — and then run for Senate in 2008.

Udall had been considered a likely gubernatorial candidate, and it was widely assumed that the Democratic nomination was his for the asking.

But he said in a statement that he became increasingly convinced that he could better serve the state in the Senate.

“My heart and my head tell me that as tempting as a run for governor is in 2006, I have a stronger desire and determination to serve in Congress and focus on the important federal issues that make America’s future uncertain,” he said. “In Congress I have established a voice on these issues, including the war in Iraq, national security, terrorism and energy independence, and, frankly, I am reluctant to set out on a different path, even if this different path makes statewide service easier to attain.”

Udall, who was elected to the House in 1998, has been looking to move up for some time. He briefly jumped into the 2004 Senate race a year ago after then-Sen. Ben Nighthorse Campbell (R-Colo.) hastily announced his retirement. But he deferred a day later to then-state Attorney General Ken Salazar (D), who wound up winning the seat in November in a key pick-up for the Democrats.

State and national party leaders had urged Udall into the 2004 Senate race for months, but he was unwilling to make the leap until Campbell announced his retirement.

“Last year, I joked about having launched the shortest U.S. Senate campaign in Colorado history; I am now embarking on what may be the longest,” Udall said.

Udall enters the 2008 Senate race under the assumption that he will be competing for an open seat: Sen. Wayne Allard (R-Colo.), who was elected in 1996, has pledged to serve only two terms. But Allard has not definitively said he will not run in 2008, and a Udall-Allard match-up would be a bruising, partisan affair.

Even if Allard does not run, Republicans are not likely to give up the Senate seat without a fight.

The GOP was bruised in Colorado in 2004, losing Campbell’s Senate seat and a House seat and yielding control of both chambers of the state Legislature to the Democrats for the first time in decades. Gov. Bill Owens (R) is term-limited in 2006 and Republicans have worried about holding on to the governorship; Owens and his allies, as reported by Roll Call earlier this week, have been urging Rep. Bob Beauprez (R-Colo.) to enter the race.

Udall’s decision not to run for governor helps the GOP some and complicates life for the Democrats. Still, Democrats have a few potentially strong candidates, including businessman and philanthropist Rutt Bridges — who also briefly ran for Senate in 2004 before deferring to Salazar — and former Denver District Attorney Bill Ritter.

Udall’s departure from his Boulder-based 2nd district seat in 2008 is likely to set off a scramble to succeed him among a half dozen state and local officials.

Udall is part of a storied Democratic family. His father, former Arizona Rep. Morris Udall (D), spent more than 30 years in Congress, and he succeeded his brother, Stewart Udall, in that seat. Stewart Udall left Congress to serve as Interior secretary under Presidents John F. Kennedy and Lyndon Johnson.

Stewart Udall’s son, Tom Udall (D), was elected to Congress in 1998 and represents New Mexico’s 3rd district.