Even before Montgomery County Executive Douglas M. Duncan formally announces his Democratic bid for Maryland governor today as planned, there are party loyalists such as C. Richard D'Amato who have already decided they're going with the other guy.

D'Amato, a former delegate from Annapolis, is one of several Democratic insiders interviewed this week who said that they like and respect Duncan -- and even think he would make a fine governor -- but that they just don't think he would match up as well against Gov. Robert L. Ehrlich Jr. (R) in the general election as would Baltimore Mayor Martin O'Malley.

"I'm going with Martin O'Malley for one main reason," D'Amato said. "Because I think he's our best hope in beating Ehrlich."

In a state where Democrats are eager to regain the governor's office that they held for almost four decades, many party insiders said that getting behind a maverick candidate, or even an underdog, is a luxury they cannot afford. What they need, D'Amato and others said, is someone they believe can win.

The view that O'Malley is more likely to be that candidate is largely a matter of geography. In 2002, Ehrlich prevailed in a heavily Democratic state by running up large margins in the Baltimore suburbs -- a region many analysts say holds the key to the 2006 contest.

Running against Democrat Kathleen Kennedy Townsend in 2002, Ehrlich won more than 61 percent of the vote in Baltimore County, where 28 percent of the voters were registered as Republicans. Similar gaps existed in Carroll and Harford counties, as well as in Anne Arundel and Howard, which are bedroom communities for Washington and Baltimore.

Duncan is largely unknown in most of that region. But many Democrats say they believe that O'Malley has the ability to compete with Ehrlich there, thanks to an expansive Baltimore media market that has put him on the evening news almost nightly for six years.

"That's part of the reason why he's selling so well, because in Baltimore County he's going to hold his own," said Del. Curtis S. Anderson (D-Baltimore), who attended a union endorsement rally in the city for O'Malley this week.

The dynamic has handed Duncan -- who trails O'Malley by double digits in early primary polling -- a difficult task.

Not only will Duncan have to convince Democrats that he has the better ideas and leadership skills, he will have to undo the perception that O'Malley is better positioned to beat Ehrlich.

"I think there are clearly a lot of Democrats out there who are so frustrated with the current administration, they would like to see everyone rally behind the one candidate who has the best chance of unseating the governor," said House Speaker Michael E. Busch (D-Anne Arundel), who is remaining neutral in the primary.

Duncan boosters argue that the counties ringing Baltimore will not be as crucial next year as they were in 2002 -- far more new voters have registered in the Washington area than anywhere else. But they also suggest that voters in suburban Baltimore will come to like Duncan once they get to know him.

"On the surface, I understand the argument" about O'Malley, said Del. Richard S. Madaleno Jr. (D-Montgomery).

"But I don't think people understand Doug's life story and how that will appeal to those in Baltimore County. Doug had very humble beginnings. In many ways, his story is very much like Ehrlich's," Madaleno said.

It is a story that Duncan will seek to highlight today during a planned announcement in front of his childhood home in the modest Twinbrook section of Rockville.

Across the state, Duncan has told prospective voters how he grew up as one of 13 children whose father taught in the public schools and whose mother worked 27 years in the county courthouse.

Ehrlich appealed to blue-collar voters by highlighting his upbringing as the son of a car salesman in Arbutus, Md.

Sen. Robert J. Garagiola (D-Montgomery), another Duncan supporter, argued that Duncan also has general election strengths that could sway some primary voters once they get to know him. Garagiola suggested that O'Malley would be far more vulnerable to Republican attacks about his stewardship.

"I think Ehrlich is more afraid of Doug Duncan, because what does he attack him on?" Garagiola said.

Republicans remain skeptical about claims that O'Malley can counter Ehrlich's popularity in Baltimore's suburbs, where Ehrlich grew up. Some also suggest that the nomination of a Democrat from Baltimore could make it easier for Ehrlich to pick up votes in the Washington region.

"I don't know that O'Malley is as popular in suburban Baltimore as he'd like to believe," GOP consultant Carol Hirschburg said.

"He's certainly more liberal than the suburbs of Baltimore," Hirschburg added.

Senate President Thomas V. Mike Miller Jr. (D-Calvert) said he believes that either Duncan or O'Malley could beat Ehrlich next year. But, Miller said, if the primary fight becomes lopsided, the candidate in trouble should step aside.

"If it becomes obvious that one is clearly the favorite," he said, "either one should step aside and say, 'Look, I care more about the future of the state than about beating up another Democrat.' "

Baltimore Mayor Martin O'Malley announces his candidacy for the Democratic nomination for governor in Baltimore on Sept. 28.Montgomery County Executive Douglas M. Duncan greets local officials during a visit last month to Cecil County to test the political waters there.