BY MARC CAPUTO AND BETH REINHARD

When it comes to fighting terrorism, Democratic Senate hopeful Betty Castor might have led with her chin and taken a beating from Republican Mel Martinez, according to a new poll showing her several points behind.

Mason-Dixon pollster Brad Coker said Thursday that Martinez leads 46 percent to 41 percent among likely voters after exploiting Castor's chief weakness: her failure to strongly denounce or fire a suspected terrorist when she was president of the University of South Florida.

Martinez's lead is within the poll's error margin of four percentage points, meaning the race could be too close to call. Another survey released Thursday, by Connecticut-based Quinnipiac University, has the race much tighter, with Martinez leading Castor 48 percent to 47 percent. That poll also suggests that terrorism is the top issue among Florida voters.

Castor was the first to bring up the terrorism issue in the general election campaign when she ran a television ad talking up her opposition to former USF Professor Sami al Arian. She suspended the professor with pay after word of his ''Death to Israel'' comments and alleged ties to terrorists were publicized.

By running the ad, Castor tried to insulate herself from future attacks. But Coker said she did the opposite.

''She led with her chin,'' he said, using a boxing term for a fighter who leaves himself open to a counterpunch.

Martinez's campaign saw it that way: It ran a TV ad featuring the former federal immigration agent who filed a Nov. 17, 1995, affidavit detailing the professor's alleged connection to the first World Trade Center bombing. In the ad, the former agent said Castor had not shown strong leadership.

Castor responded with an ad from the former USF police chief, who said Castor did enough.

Coker said it was too little too late.

''On one side you have a federal agent, and on the other you have the chief of campus police. It's not as credible,'' Coker said. ``I remember the chief on my college campus as the guy sitting in a golf cart sipping coffee and watching the pretty girls go by. He wasn't an expert on terrorism.''

Coker said Martinez is also benefiting from his immigrant-made-good story, which has earned him a 46 percent favorableness rating, according to the poll of 625 likely voters. About 23 percent view him unfavorably. Castor's favorable/unfavorable percentages are 36/26.

Martinez saw the biggest jump in favorable ratings -- 20 percentage points compared to Castor's four -- since Mason-Dixon last polled the matchup before the Aug. 31 primaries.

Quinnipiac pollster Clay F. Richards said more than a third of respondents to the university's poll said they didn't know enough of the candidates, in part, because the race has been overshadowed by the presidential contest and recent hurricanes.

Richards said the two are locked in a neck-and-neck race among likely voters, with Martinez leading 48-47. Castor led a month ago, 43-42. When Quinnipiac accounted for newly registered voters, Castor led 44-40, which is within the error margin of three percentage points.

The two polls differed on who held the edge across North Florida. Quinnipiac showed Castor leading it, and Mason-Dixon said Martinez was running strongest there. Both polls showed Martinez leading in the Gulf Coast and Central Florida, and Castor leading in Southeast Florida -- an area that's rich with Democrats.

MARTINEZ CAMP

Martinez's antiterrorism ads have run heavily in South Florida.

''She treated this like something out of Animal House and put [Arian] on what amounts to probation,'' said Martinez media guru Stuart Stevens. ``She could have done more.''

Neither Stevens nor Martinez have been able to answer why, if Arian was such a threat, he campaigned with President Bush in 2000 and later attended a White House briefing with Bush strategist Karl Rove. Martinez said such questions ``are beside the point.''

For her part, Castor has been at pains to explain why she didn't denounce Arian often or attempt to fire him, even if the university lost the case. Her spokesman, Dan McLaughlin, points out that the federal government did less than Castor by failing to indict Arian until recently.

McLaughlin also questioned the credibility of immigration agent William West, who appears in the Martinez ad that mentions two other suspected terrorists who worked at USF.

McLaughlin points out that West had trouble convincing a judge of the guilt of the men -- who were subsequently freed -- and displayed limited knowledge of Middle Eastern affairs.

''This is more nasty, filthy lies on the part of Martinez,'' McLaughlin said. ``When the truth comes out, it's not going to help Martinez on Election Day.''