President-Elect Barack Obama ’s decision to name Democratic Sen. Ken Salazar of Colorado as his Interior Secretary means that Democrats may have more of a fight to hang on to the seat than if Salazar ran for re-election in 2010.

Salazar would have been decisively favored to win in a state that has trended Democratic in recent elections.

But the seat will be little tougher for the Democrats to hold with Salazar not on the ballot. Whereas Salazar would have run for re-election as a six-year incumbent, the person that Democratic Gov. Bill Ritter appoints will have served less than two years by the time of the 2010 election, when he or she presumably will seek a full six-year term.

Potential Democratic appointees include Denver mayor John Hickenlooper, outgoing state House Speaker Andrew Romanoff and three members of Colorado’s U.S. House delegation: Diana DeGette , who represents the Denver-based 1st District; John Salazar , the senator’s older brother, who represents a mostly rural district in western and southern Colorado; and Ed Perlmutter , who just won a second term representing suburbs west, north and east of Denver.

DeGette, a liberal who is the most senior member of Colorado’s seven-member House contingent, easily won a seventh term last month in a heavily Democratic district. She would become Colorado’s first woman senator.

Salazar, a centrist Democrat, easily won a third term in a district that narrowly backed John McCain in the Nov. 4 election. His selection would ensure that a Hispanic continues to serve in that seat.

Perlmutter is a former state senator who was first elected in 2006 by a comfortable margin and then overwhelmingly re-elected last month in a district that was drawn after the 2000 census to be highly competitive.

Romanoff, who was barred by a state term limits law from seeking re-election to the legislature, is being considered for the Colorado Secretary of State job, which was vacated by Republican Rep.-elect Mike Coffman.

Any of the Democrats who will be passed over for the Senate appointment could challenge Ritter’s choice in a primary election, though that is unlikely.

The Republican bench of candidates in thinner, in part because Democrats have dominated recent elections in Colorado — including Salazar’s win in 2004 over brewing company executive Pete Coors, Ritter’s landslide election as governor in 2006 and Democratic Rep. Mark Udall ’s easy win last month over former Rep. Bob Schaffer for the seat of retiring Republican Sen. Wayne Allard . Barack Obama defeated John McCain by 9 percentage points in the presidential balloting.

Potential Republican candidates include two former House members who have expressed some interest in returning to political life — Bob Beauprez, who lost decisively to Ritter in the 2006 election for governor, and Scott McInnis, who served for a dozen years in the seat that John Salazar now holds. Republican Rep. Tom Tancredo , a conservative who is retiring at the end of this year from a district that takes in suburbs south and west of Denver, also is mentioned as a possible candidate.

Including Obama, Vice President-elect Joseph R. Biden Jr. , and Clinton, who is the Secretary of State designate, Salazar is the fourth Democratic senator who will resign — or has done so — to join the new presidential administration.

Obama is the only one among that quartet who has formally vacated his seat. The Democratic-run state legislature has initiated an impeachment inquiry into Blagojevich, who was arrested last week on charges that he used his Senate appointment power to secure lucrative campaign contributions and other benefits for himself and his wife.