When former Representative Barbara Lee announced that she would run for mayor of Oakland, Calif., many considered her victory to be a sure thing.

After all, Ms. Lee had represented the city for more than a quarter-century in Congress and in the State Legislature before that. She gained national fame as a progressive lawmaker who was the only member of Congress to reject the use of force after the Sept. 11 attacks, reasoning that it gave the president too much war authority.

To Ms. Lee’s supporters, her return home was seen as a worthy bookend to her career, with a chance to rescue her hometown from its doldrums and political decay. Oakland, a city of 436,000 residents across the bay from San Francisco, has suffered mightily since the pandemic as it has faced soaring crime rates, the departure of its last major sports team and a yawning budget deficit.

The city had a sudden vacancy at the top because voters ousted Mayor Sheng Thao in November, a sign of their frustrations with the city’s direction and with a dark legal cloud over Ms. Thao.

But Ms. Lee, 78, is no longer considered a shoo-in to win Tuesday’s election.

Loren Taylor, a former Oakland City Council member, has gained traction with voters as a moderate candidate who has pledged to fix potholes, reduce crime and shorten 911 response times.

Ms. Lee is still considered the heavy favorite in the field of 10 candidates. But Mr. Taylor, 47, recently surpassed her in fund-raising and landed the endorsement of the editorial board at The San Francisco Chronicle. “Oakland is broken,” Mr. Taylor often says on the campaign trail, a message that has resonated with some voters.

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