The other five Democratic U.S. Senate candidates who sat on a riserat the University of Maryland at College Park last week seemed to have the same mission: stick it to Rep. Benjamin L. Cardin.

University professor Allan Lichtman and socialist activist A. Robert Kaufman attacked Cardin for his stand on the Iraq war, while businessman Josh Rales knocked him for being too much of a Capitol Hill insider.

"If we keep electing the same people, we should expect to keep getting the same results," Rales told the audience in the student union.

Cardin shrugged off the criticism, but the jabs and volleys were a reminder of the unique challenge he faces as he seeks the Democratic nomination for an open Senate seat. Even though Cardin remains locked in a tight contest with former NAACP president Kweisi Mfume, the efforts of 16 long-shot Democratic contenders could ultimately determine who wins the Sept. 12 primary.

Political analysts are calling it the "Nader effect" in the Maryland primary, a reference to Ralph Nader, whose third-party bid left some Democrats calling him the spoiler in the 2000 presidential election.

"They may take very small percentages of the vote, but the fact is that Cardin's race with Mfume is so close that they could make the difference," said Matthew Crenson, a political science professor at Johns Hopkins University.

Crenson said the "spoiler candidates" are taking more votes from Cardin because they, like Cardin, are white. Mfume benefits from being the prominent black candidate in a primary in which as many as 40 percent of voters could be African American.

"Clearly, they're subtracting from Cardin's coalition," said Thomas F. Schaller, a political science professor at the University of Maryland Baltimore County who is active in Democratic politics and supports Cardin. "The only question is how much."

In a Washington Post poll last month, 13 percent of registered Democrats opted for a candidate other than Cardin or Mfume, including 5 percent who backed former Baltimore county executive Dennis Rasmussen, a candidate whose campaigning has largely been confined to Cardin's home county. Rasmussen's Web site lampoons Cardin as someone who thinks he is entitled to a Senate seat because it's "his turn."

Mike Schaefer, a Baltimore area businessman, has been erecting "Schaefer for Senate" signs across the state, a move that could capitalize on voter confusion over his surname. It's the same as that of William Donald Schaefer, the former four-term Baltimore mayor and two-term governor who is running for reelection as state comptroller. Mike Schaefer, a political neophyte, has a Web site that includes a top 10 list of reasons to vote against Cardin, including: "After 20 years in Congress, America's gotten all his great ideas already. The new term will be ho-hum."

Of greatest concern to Cardin, though, is Rales, political analysts say. A millionaire from Montgomery County who is largely self-financing his campaign, Rales began saturating the airwaves with television ads July 5.

Records compiled by The Post show that Rales, who was backed by 1 percent of voters in the recent poll, has spent at least $2.1 million to run more than 1,800 30-second spots aimed at introducing him to Maryland voters in the Washington and Baltimore media markets. Cardin has yet to air ads.

Jim Jordan, a former executive director of the Democratic Senatorial Campaign Committee in Washington, said it's "probably unknowable at this point" how much damage the second-tier candidates will do to Cardin.

But he said it can be "nerve-racking" for a candidate when an opponent is on television and he is not.

Cardin's longevity in politics has provided him plenty of advantages. Long-standing relationships built over 20 years in the Maryland House of Delegates and 20 years in Congress helped him garner early endorsements across the state. He far surpasses the rest of the Democratic field in fundraising, a factor that could help him connect with voters late in a contest in which a third of the electorate is undecided.

And Jordan said the fundraising will probably produce a base of voters more likely to turn out in a primary than those courted by second-tier candidates.

But Cardin's long political career also has made him, more than Mfume, the target of persistent criticism from his rivals. With longtime Democratic Sen. Paul S. Sarbanes retiring, the other contenders are essentially treating Cardin as the incumbent.

At last week's debate, as they have in numerous panel discussions across the state, Lichtman and Kaufman pounced on Cardin's vote to authorize funding for the war in Iraq.

"Ben Cardin may have voted against the war two years ago, but he voted to finance the war when Bush asked him to, and I find that unforgivable," said Kaufman, who along with Lichtman has support from 3 percent of registered Democrats, according to the Post poll.

Cardin shot back, saying he was unashamed that he voted to "support the troops."

"My record's clear on the war, and I ask you all to judge me on it," he said.

Afterward, Cardin said all the jabs "come with the turf."

"We'll have our chance to communicate to the voters," he said. "We believe our record will appeal in November."

Cardin has reserved $680,000 in television time for the final weeks of the campaign. That should help him contend with Mfume, who held a slight lead over Cardin in the June poll but at present lacks the funds to pay for a sustained presence on television.

Mfume, though, said he does not believe he's benefiting appreciably from the presence of so many other candidates in the race. "I think they're helping themselves more than they're helping me," he said.

Rales and Lichtman said they hope Mfume is right about that.

"I do not look at myself as a Ralph Nader," Lichtman said. Rales agreed: "The only thing that would spoil this election is if we didn't give voters a choice."