John C. Liu, the city comptroller, has suffered a steep drop in his approval ratings in the wake of a federal investigation into his campaign fund-raising, according to a Quinnipiac University poll released on Thursday.

The poll found that only 25 percent of New York City voters said they think that Mr. Liu would be a good mayor, compared with 46 percent who said he would not be. The remainder had no opinion on the question. Only 28 percent said he should resign, and 43 percent said he should not.

The poll represents the first evidence that the scandal has taken a toll on Mr. Liu’s public standing.

Last month, federal authorities arrested Xing Wu Pan, a fund-raiser for Mr. Liu, on charges of attempted wire fraud and conspiracy to commit wire fraud, saying he had sought to help an F.B.I. agent posing as a businessman circumvent campaign contribution limits. An examination by The New York Times has also raised questions about the legality of donations to Mr. Liu, including whether some of the donors had been reimbursed for their contributions.

Mr. Liu also failed to comply, The Times found, with a city requirement that he disclose the names of so-called bundlers — well-connected people who collect contributions for a candidate from friends, relatives, business associates and others. Mr. Liu, after some initial reluctance, has pledged to disclose the identities of those bundlers, but the process is taking longer than his campaign had anticipated.

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