Former state Sen. Mark Leno appears to be the out-of-the-gate favorite in San Francisco’s upcoming mayoral race — but with less than a month on the job, acting Mayor London Breed is hot on his heels, according to the first polling in the race.

A phone survey of 627 San Francisco registered voters taken last month shows Leno clocking in with 26 percent of first-place votes in the June ranked-choice election.

Breed — the Board of Supervisors president who has been acting mayor since Ed Lee died last month — has 20 percent. None of the other declared or rumored candidates is above 11 percent.

“Frankly, I was surprised to see that this is shaping up as a two-person race from the start,” said political consultant Jim Stearns, who included the mayoral question in a poll he commissioned for a progressive organization that is considering putting an unrelated measure on the ballot.

“It’s a snapshot, and it could change,” he said. “But with the race being a short, five-month shot to the finish line, it’s a big head start for Leno and Breed.”

Leno has already declared his candidacy. Breed has not, but is widely believed to be running.

The Public Policy Polling survey was conducted Dec. 18 and 19, and has a margin of error of four percentage points.

The poll shows Democratic state Assemblyman David Chiu with 11 percent of first-choice votes, followed by City Attorney Dennis Herrera at 10 percent. Neither has entered the race.

Supervisor Jane Kim, who has indicated she’s running, and Assessor-Recorder Carmen Chu, who has not, each has 5 percent.

Former San Francisco Supervisor Angela Alioto declared her candidacy after the poll was conducted and was not included. Candidates have until Jan. 9 to enter the contest.

The poll found that 59 percent of voters have a favorable opinion of Leno, the highest total among any of the potential candidates. Herrera’s favorables are 49 percent, and Breed’s are 43 percent.

Leno, who has been running for mayor for close to a year, reports having raised $400,000 for the race, giving him a significant advantage over the other candidates. But if Breed decides to run, she’ll have the backing of tech investor Ron Conway, who has a track record of raising hundreds of thousands of dollars in short order to elect candidates.

So, Leno and Breed will be pretty well matched.

Jackie jump? Word that Peninsula Rep. Jackie Speier is mulling a run for governor has grabbed the attention of the early front-runners in the race — Lt. Gov. Gavin Newsom and former Los Angeles Mayor Antonio Villaraigosa.

The Hillsborough Democrat, a prominent Bay Area figure since her days as a staff aide to Rep. Leo Ryan in the 1970s, could cut into Newsom’s Northern California base of support.

“The whole question is who comes in second to Gavin” in the June top-two primary, said one Speier confidant, speaking on condition of anonymity because the congresswoman has not made a decision. “If it’s Jackie, then he has a problem.”

Speier has been making headlines of late as a congressional leader of the “MeToo” anti-sexual harassment campaign.

A natural talker who is not afraid to take on sensitive issues, Speier would also inject life into what has become a gubernatorial road show of sleeper debates between Newsom, Villaraigosa and the other candidates.

However, her family is known to be tired of her long-distance commute to D.C. and has been holding out hope that Speier, 67, would consider leaving politics rather than sign up for four or eight years in Sacramento.

Newsom supporters also point to the $20 million war chest he has already amassed, noting it would hard for Speier to match on short order. But our Speier confidant figures that wouldn’t be a problem:

“She is not going to need anything near that (amount), because this is the year of the woman.”

Willie return? A new poll shows San Francisco voters divided on whether a “caretaker” should occupy the mayor’s office until the June election — but that if a short-timer gets the job, their top pick is former Mayor Willie Brown.

“You’re kidding,” Brown said when he stopped laughing at the news.

Brown, who served two terms as mayor and now writes a weekly column for The Chronicle, is on record saying the job should go to someone who wants it full-time.

A survey by Public Policy Polling of 627 registered San Francisco voters taken last month shows Brown would be the first choice of 30 percent of respondents if the Board of Supervisors goes with a caretaker. Others whose names have been floated run well behind, including former Supervisor and state Assemblyman Tom Ammiano (19 percent) and ex-Mayor Art Agnos (13 percent).

City Hall insiders are hotly debating whether a candidate should be allowed to serve as interim mayor, and the public also appears to be divided.

The poll found 39 percent of those surveyed say the interim mayor should not be a candidate for the job, and 34 percent say he or she should run for mayor. The rest aren’t sure.

Brown, for one, is sure of something — he has no interest in returning to Room 200.

“I wasn’t all that good when I had the job,” the 83-year-old Brown said. “And I don’t think I would be any better now.”

But he added, “You can do me one favor — make sure Art (Agnos) sees the poll numbers.”

San Francisco Chronicle columnists Phillip Matier and Andrew Ross appear Sundays, Mondays and Wednesdays. Matier can be seen on the KPIX TV morning and evening news. He can also be heard on KCBS radio Monday through Friday at 7:50 a.m. and 5:50 p.m. Got a tip? Call (415) 777-8815, or email matierandross@sfchronicle.com. Twitter: @matierandross