Congressman Mark Udall made it (almost) official today: He is filing his paperwork to run for the U.S. Senate in 2008. As the Rocky Mountain News reports:

Rep. Mark Udall officially entered the 2008 U.S. Senate race on Monday by filing campaign finance papers with the Secretary of the U.S. Senate.

Udall, D-Eldorado Springs, plans an official campaign announcement later this year, but he has long been positioning himself to run for the seat being vacated by the retiring Sen. Wayne Allard, R-Loveland.

On Monday, the deadline for filing first-quarter campaign finance reports, Udall filed paperwork to form a new Senate campaign committee, “Udall for Colorado.”

He also reported raising more than $1.5 million for his House of Representatives campaign account, including $334,882 in the first quarter of 2007. That account could be transferred to the new Senate campaign committee.

Udall has jokingly said that he is running one of the longest Senate campaigns in history, which may be true given that he made his intentions known in early 2005. He was briefly a candidate for the Senate in 2004 before giving way to Ken Salazar, and he declined to run for governor in 2006, so while today’s announcement may seem like a no-brainer, it actually does mean quite a bit.

I would expect that Udall will have the Democratic field to himself in 2008, so the only mystery left is about who will run on the Republican side. All signs currently point to former candidate Bob Schaffer, who lost in a GOP primary to Pete Coors in 2004, but moderate Republicans may look for another candidate instead of the conservative Schaffer. Whether Republicans have a primary or not, Udall is the prohibitive favorite to win the seat in 2008.