U.S. Sen. Frank Lautenberg (D., N.J.) is leading Republican challenger Dick Zimmer, but voters remain concerned about Lautenberg's age, and fewer than half approve of how he's doing his job, according to a poll released yesterday.

Asked whom they would vote for, poll respondents chose Lautenberg over Zimmer, a former congressman, 47 percent to 38 percent, according to the Quinnipiac University poll.

In a separate question, 46 percent of those polled thought Lautenberg was doing a good job. He gets the modest approval rating because he "has been around a long time, made some political enemies, and there are people who just don't like him," said Clay Richards, the poll's assistant director.

But Richards said the four-term senator's "biggest advantage - as always - is a lack of a serious challenger, and that is the case again."

He said Zimmer was not a serious candidate - yet - because 67 percent of the poll respondents said they didn't know enough about him to form an opinion.

Zimmer spokesman Ken Kurson took a brighter view of the poll, saying that, although the candidate isn't well known, most of those who do know him like him. Twenty-one percent of those polled said they had a favorable impression of Zimmer; 11 percent had an unfavorable impression.

New Jersey voters last elected a Republican to the Senate in 1972.

Lautenberg survived a bruising primary race against Camden County Democratic U.S. Rep. Robert E. Andrews, spending $3 million to turn him back.

"Once voters begin tuning in after Labor Day, they will recognize how hard the senator has fought for them," said Julie Roginsky, a Lautenberg spokeswoman.

Lautenberg's campaign says he beat back the age issue when he won the June 3 primary, but voters still seem concerned. The poll found that 54 percent said he was too old to be effective in another six-year term.

Lautenberg is 84 and would be 90 when the next Senate term ends. The poll of 1,473 New Jersey voters was conducted June 5-8 and had a margin of error of plus or minus 2.6 percentage points.

Summer polls generally show that voters like to "flirt" with Republicans in statewide races but by November they support Democrats, said Patrick Murray, director of the Monmouth University Polling Institute.

Contact staff writer Cynthia Burton at 856-779-3858 or cburton@phillynews.com.