The 20-year congressman will face Lt. Gov. Michael Steele in November's general election to succeed retiring five-term Democratic Sen. Paul Sarbanes.

A litany of problems at Maryland polling places, including voting machines that remained down until late Tuesday morning in the Washington suburbs, forced thousands of voters to cast provisional ballots and kept some polls open an hour later than scheduled under an order from judges in Baltimore and Montgomery County.

In a long but amicable race with Mfume, Cardin portrayed himself as an experienced legislator, playing up his support for universal healthcare and his vote against authorizing President Bush to go to war in Iraq, at a time when such a stance was politically unpopular.

"The good news for Cardin is that the primary was very tame," says Jennifer Duffy, a Senate race analyst with the Cook Political Report. "He didn't pick up a lot of baggage. There was not a single negative ad."

Backed by the Democratic establishment, Cardin raised $5.5 million to Mfume's $1.1 million, though Mfume's support in Maryland's sizable black community made him a formidable opponent.

"Mfume is a victim of not being able to raise the money needed to be competitive," says Duffy. "Whether you blame that on the Democratic leadership for endorsing Cardin or whether Mfume contributed by not working hard enough, I don't have the answer."

In a sprawling Democratic field, Cardin and Mfume were long considered the frontrunners. Mfume's fortunes appeared to have been tied to the prospect of second-tier candidates like businessman Josh Rales siphoning off votes from Cardin.

"The Rales-plus-others contingent of candidates needed to get a combined 20 percent to put Cardin in jeopardy, but they didn't," says Tom Schaller, an associate professor of political science at University of Maryland-Baltimore County.

Preprimary polls predicted that Cardin would handily defeat Steele in a general election matchup, while forecasting an Mfume-Steele contest to be a much tighter race. But Steele, the first African-American to be elected statewide in Maryland, has raised $5 million and has vowed to reach out to traditionally Democratic black voters, who make up nearly 30 percent of the state electorate.

"Michael's message of empowerment and opportunity is resonating across Maryland," Elizabeth Dole, National Republican Senatorial Committee chair, said in her statement congratulating Steele on his win in the Republican primary, in which he faced no serious opposition.

Historically, black Republican candidates have failed to translate into black Republican votes. "But there is the whole issue of African-American voters being unhappy with the [Democratic] Party establishment for having anointed Cardin," says Duffy. "He could get enough of them to make a difference."

Steele has worked hard to distance himself from the Republican Party, snagging an endorsement from hip-hop mogul Russell Simmons, who has traditionally supported Democrats, and unveiling a TV ad last week called "building bridges." Last night, in a show of bipartisanship, Steele took the unusual step of visiting the headquarters of both the incumbent Democratic county executive and his Democratic primary challenger in the Washington suburb of Prince George's County, where Steele lives.