CASTLE ROCK, Colo. | When Michael Bennet was tapped to fill Colorado's U.S. Senate vacancy in January, it didn't sit well with Diane Rubenstein.
Like many Colorado Democrats, she had expected the governor to name former state House Speaker Andrew Romanoff, an eight-year legislative veteran and rising star in the party, not the lesser-known, politically untested Mr. Bennet, a former superintendent of Denver's public schools.
"I didn't know a single person who wanted Bennet," Ms. Rubenstein said at the Douglas County Democratic Party's Oktoberfest late last month. "We didn't know why Bennet had been chosen, and we want the person we choose to fill that seat."
She may get her wish. Mr. Romanoff bucked the state and national Democratic establishment and added another layer of uncertainty to the party's 2010 election prospects by announcing Sept. 16 that he will challenge Mr. Bennet for the Democratic nomination.
Mr. Romanoff's candidacy all but ensures a bruising, expensive primary battle, exactly what Democrats had hoped to avoid as they scramble to hold their 2008 gains. For someone who just upended his party's best-laid plans, however, Mr. Romanoff is decidedly unapologetic.
"I respect the governor's right to fill a vacancy," Mr. Romanoff said. "But now the voters of the state should have a chance to have their say. We have one million Democrats in this state and three million voters in the state of Colorado, and only one has cast a ballot."
Still, it's not easy running against an incumbent, and Mr. Bennet, who replaced Sen. Ken Salazar after he accepted President Obama's offer to become interior secretary, would appear to have the early advantages. National Democrats have thrown their support behind him by helping him amass a war chest of $2.6 million, which was supposed to scare off potential challengers.
The White House reportedly attempted to lure Mr. Romanoff out of the race with a high-profile federal job, although a spokesman told the Denver Post that no specific offer was made. The day after Mr. Romanoff announced his candidacy, Mr. Obama formally endorsed Mr. Bennet.
Before Gov. Bill Ritter Jr. named him to the Senate seat, Mr. Bennet served three years as Denver Public Schools superintendent. He received glowing reviews in that post and had been under consideration for education secretary in the Obama Cabinet. Before that, he was Denver Mayor John W. Hickenlooper's chief of staff.
Mr. Bennet is the descendant of Democratic Party bluebloods: His father, Douglas J. Bennet, held several posts in the Carter and Clinton administrations, and his grandfather, Douglas Bennet, was an economic adviser to President Franklin D. Roosevelt.
But Mr. Bennet has never run for office, a void in his resume that instantly raised red flags among Democrats and created an opening for the state's limping Republicans.
Mr. Romanoff had been the obvious pick for the seat. Term limits left him the highest-ranking Democrat in the state without a job, and his popularity was second only to Mr. Hickenlooper's among the party's rank and file.
"This happened because Bennet was an improbable choice," said Denver pollster Floyd Ciruli. "Bennet was not on anyone's radar screen — he had no party experience, no electoral experience. I think that left a significant amount of bitterness and division in the party."
In the eight months since assuming office, Mr. Bennet has acquitted himself respectably, traveling extensively throughout the state. He bolstered his moderate bona fides by aligning with the fiscally conservative Blue Dog Democrats in the house, but he hasn't been able to match Mr. Romanoff's statewide name recognition or status among the party faithful, those most apt to vote in primaries.
"All the polls show voters still have no identification with [Mr. Bennet]. They don't know who he is," Mr. Ciruli said. "Hence there was this tremendous vacuum. This is going to produce what will no doubt be a difficult primary. It greatly damages Bennet, who was vulnerable to start with."
Still pinching themselves to make sure they're not dreaming are state Republicans, who suddenly have a real shot at the once-impregnable Salazar seat. The Republican Party is quickly accruing its own crowded primary field, a Snow-White-and-the-Seven-Dwarfs affair led by former Lt. Gov. Jane Norton, whose sister is married to veteran Republican political operative Charlie Black.
Mrs. Norton won the straw poll with 109 votes at the state Republican Party's retreat last month at the Keystone Resort. Close behind her were Weld County District Attorney Ken Buck and Aurora City Council member Ryan Frazier, with 94 votes each.
Election experience aside, the biggest challenge in the Bennet-Romanoff matchup may be finding significant distinctions between the candidates. They are nearly the same age — Mr. Bennet is 45, Mr. Romanoff is 43 — and both position themselves as non-ideological, pragmatic problem-solvers. Both have the same elite East Coast academic credentials — Mr. Bennet holds a law degree from Yale, Mr. Romanoff has degrees from both Yale and Harvard — not normally found among Colorado lawmakers.
Neither can credibly wear cowboy boots, which could prove troublesome in the general election. Despite Colorado's recent tilt to the left, the Democratic Party's biggest success stories over the past two decades, including those of Mr. Salazar, Sen. Mark Udall and Gov. Roy Romer, were closely identified with the rural West or could trace their Western roots back generations.
At the Sept. 26 Oktoberfest, Democrat Harisha Bastiampillai said he was still weighing the merits of the candidates.
"There are people who don't feel like Bennet is a dynamic enough campaigner," he said. "But I know Bennet is highly regarded as an intellectual and did a great job with [the public schools]. Romanoff is more identified with Colorado."
Democrat Bob Thomas, a retired military officer, said he worried at first that Mr. Romanoff's candidacy would hurt the party's prospects.
"At first I was thinking, 'We don't need this, we need unity,'" Mr. Thomas said. "But now I think it'll be healthy. As Democrats, we can't lose with either one."