Ever since Sen. Paul Sarbanes (D-Md.) set off a giant game of political dominoes in Maryland by announcing that he would not seek re-election, Rep. Chris Van Hollen’s (D-Md.) domino was considered one of the biggest.

But after weeks of rampant and often contradictory speculation among national and state political insiders, the sophomore Congressman announced Monday that he was staying out of the game and would run for re-election instead of the Senate in 2006.

“It was a difficult decision because I believe we could have waged an energetic and ultimately successful campaign,” Van Hollen wrote in an e-mail to supporters.

Van Hollen’s decision leaves Rep. Benjamin Cardin (D-Md.) as the heavy favorite in what could have been a free-for-all Democratic primary.

For now, Cardin will face former Rep. Kweisi Mfume (D), a candidate with far more charisma and greater oratorical skills than the veteran Congressman, but who may have been damaged by sluggish fundraising and allegations of favoritism during his nine-year tenure as president of the NAACP.

“This is incredibly positive news for Ben Cardin — there’s no other way to interpret it,” said Keith Haller, president of Potomac Inc., an independent polling firm in Bethesda, Md. “He has been looking for the one-on-one, head-to-head primary.”

Ever since the Senate race began to take shape after Sarbanes’ March retirement announcement, Mfume has all but admitted that he would rather run in a crowded primary.

Van Hollen’s announcement also represents a victory for party leaders, led by House Minority Whip Steny Hoyer (D-Md.), who have been trying to tilt the Democratic contest to Cardin.

“This is a big day for Steny as the Democrats’ true kingmaker” in Maryland, Haller said.

Several leading state and national Democrats were seeking to avoid a bloody primary, painfully aware that whomever the party nominates will square off against Lt. Gov. Michael Steele (R), who is certain to receive intense support from Republicans in the state and nationally.

Although Steele is officially only exploring a Senate bid, the fact that White House Deputy Chief of Staff Karl Rove will headline a fundraiser for him in two weeks is a clear sign of his enormous fundraising potential.

Not that Cardin has been a fundraising slouch; he has raised more than $1 million since announcing his Senate candidacy in April. It heartens Democratic leaders to think that Cardin will not have to deplete his campaign treasury in a primary race with Van Hollen, who was also awash in cash.

But Cardin’s march to the nomination is not a foregone conclusion.

Mfume has been buffeted by a string of embarrassing headlines about his time leading the NAACP and took in about $150,000 since entering the Senate race in March. But he’s a gifted campaigner who appeals to many segments of the Democratic base.

Van Hollen’s departure from the race means his core supporters in vote-rich Montgomery County are up for grabs, and both Cardin and Mfume are certain to make appeals to those active, wealthy and well-educated Democrats.

What’s more, Democrats in Maryland are under heavy pressure to promote black candidates for statewide office — an imperative highlighted by Steele’s swift political rise.

Kevin Igoe, a GOP consultant in Maryland, said Cardin’s path to Senate victory is filled with several potential race-based hazards since he, like all Democrats, must generate a large black turn-out in the general election.

“Cardin has to pull off an unprecedented feat,” Igoe said. “He has to defeat two qualified African-American candidates, relying on the supporters of the first to beat the second.”

Even so, the Democratic field is not completely set yet. Anne Arundel County Executive Janet Owens, psychiatrist Lise Van Susteren — the sister of Fox News personality Greta Van Susteren — and developer Joshua Rales continue to ponder joining the Senate race.

Owens, who is weighing running for several offices in 2006, is on vacation this week and could not be reached for comment Monday. Van Susteren did not respond to a phone message left at her office, but several Democratic insiders believe she is going to run.

Rales said his decision will not be influenced by Van Hollen’s, and said he will try to determine by the end of the summer whether he can put together a strong enough campaign team to run a competitive race.

“If I’m going to be on the stump urging the government to be better managed, I’d better be sure I’m going to have a well-managed campaign,” he said. “You especially need that if you’re an outsider.”

Meanwhile, Van Hollen’s decision to seek a third term staves off what was certain to be a wild primary in the 8th district to succeed him.