Arlington County Democrats will participate in an open caucus Tuesday to select a candidate to fill the seat of former board chairman Charles P. Monroe, who died Jan. 11 of a brain aneurysm.

The winner of the caucus will represent the Democratic Party in the special election on March 11.

The race has become unusually competitive, party officials and political observers said. The three candidates have radically different backgrounds yet appear equally qualified and adept at rallying constituencies to turn out in heavy numbers for Tuesday's vote, observers said.

In addition, the race has opened deep fissures in the Democratic Party. Some loyalists argue that the party should embrace new leadership, while others say a veteran candidate would be more suitable. Other party members want a minority candidate on the ticket.

Candidates for the Democratic nomination are activist Christian Dorsey, an African American who unsuccessfully challenged then-board chairman Chris Zimmerman last year; Walter Tejada, a longtime civic activist and member of the county's fiscal affairs committee, who hopes to become the county's first Latino board member; and former board chairman Albert C. Eisenberg, who has never lost an Arlington board election.

Mike W. Clancy, 45, who has mounted two unsuccessful campaigns for the board, was chosen last week by the county Republican Party as its nominee.

Some Democratic officials and political observers admit that the race among Eisenberg, Tejada and Dorsey has a lot of people scratching their heads, wondering who might best serve the county of 189,000.

"There really is no bad candidate," said Todd Endo, a community activist and director of the Urban Alternative, a Columbia Pike advocacy group. He is supporting Dorsey. "I could make an argument for why all three would be good choices."

What makes the race particularly unpredictable and tense, some political observers said, is that each candidate has a strong base.

Eisenberg, 56, has more than two decades of political experience in the county. He was first elected to the board in 1983 and has been on a slew of commissions and task forces in more than 25 years of county service.

Eisenberg and his supporters said his 15 years on the board and three terms as chairman are invaluable, particularly in a five-week campaign.

Some Democrats have publicly expressed concern about the party's history in special elections. Democrats lost two of the last three special elections, the last in 1999 when Republican Mike Lane defeated Monroe.

"We should have someone in there who is a proven vote-getter," said county Treasurer Francis X. O'Leary, who is supporting Eisenberg.

O'Leary upset some Dorsey and Tejada supporters last week when he used the county e-mail system to message party insiders, encouraging them to vote for Eisenberg.

County Attorney Stephen MacIsaac said O'Leary did not violate county rules by sending the e-mail because rules on using the system apply only to county employees. "He's a constitutional officer," MacIsaac said. But he added, "It's the county's equipment, and we'll probably want to talk with Frank to have him abide to [county policy]."

Although some party members cried foul, O'Leary said that it was a mistake and that he meant to use his personal e-mail account.

Still, he makes no apologies about campaigning for Eisenberg.

"We've lost a few of these elections in the past," O'Leary said in an interview. "I think we need someone who is battle-tested to serve out the remainder of Charles's term."

Eisenberg, vice president for government relations for the Greater Washington Board of Trade said he thinks that in the short campaign season, "it's really about electability . . . but it's also about experience. We've got some difficult times ahead with the budget and other issues, and it's important for someone to be able to hit the ground running."

Supporters of Dorsey, 31, and Tejada, 45, said they would provide a fresh look at issues that concern all Arlingtonians. They also said that because there is so much turnover among registered voters in the county -- about 11 percent a year, according to party officials -- Eisenberg's experience holds no cachet for many potential voters next month and therefore does not guarantee Democrats a victory.

"We see Walter as the perfect candidate because he has fresh ideas, and he's also known around the county," said Ed Fendley, Tejada's campaign manager.

But there is also concern in many political circles that Monroe, who was black, should be succeeded by a minority board member. While Tejada and Dorsey have stressed that they would represent all Arlingtonians, they also think the board membership should reflect the county's diversity.

Minorities make up about 41 percent of the county population.

Fendley said Tejada, who works as a consultant to several area banks, has a broad base of support, including an endorsement from Zimmerman and a growing base of Hispanic residents quietly becoming a voting bloc.

Hispanics make up nearly 19 percent of the county population, according to the 2000 Census, but it is unclear how many are eligible to vote.

Others believe that Dorsey, a member of the Arlington Tenant-Landlord Commission and a director of a children's literacy program, would best carry on Monroe's vision.

"I think a lot of people in the [black] community feel as if none of the current board members really will further Charles's vision," said Darnell Carpenter, a Democratic committee member and a Dorsey supporter. "There's lots of feeling that the seat should be filled by another African American."

Dorsey's supporters said he has proven that he can mobilize voters who support his agenda, which focuses largely on providing more affordable housing. Dorsey won 40 percent of the vote in his primary challenge against Zimmerman in June. Much of his support came from disaffected Democrats concerned about some of Zimmerman's stands on affordable housing.

Dorsey is looking to rally the African American vote on top of that base. He has received support from many leaders in the historically black neighborhoods of Nauck and Hall's Hill, including civic activist Alfred Taylor and county NAACP president Talmidge Williams.

In an open letter this week, several black leaders said: "Christian Dorsey has long been a proponent of the ideals that Charles espoused." Party leaders, who have publicly distanced themselves from O'Leary's e-mail, said any of the candidates could win.

"I think you'll find that each candidate has considerable strengths," said Dan Steen, county Democratic Party chairman. "We'll just have to wait and let it play out."