Former San Jose Mayor Sam Liccardo has maintained a first-place lead in the race to replace retiring Rep. Anna Eshoo of Menlo Park, but the contest for second place has been far more volatile, with Santa Clara County Supervisor Joe Simitian and Assemblymember Evan Low of Campbell repeatedly swapping spots. All three are Democrats.

On Monday afternoon — nearly three weeks after election day — Simitian’s lead over Low narrowed to a single vote. At one point last week, Low led Simitian by two votes. The top two finishers will advance to the November general election.

Having a competitive race this close for a major office is “shockingly unusual,” said Paul Mitchell, a Democratic strategist and political data expert.

The current state of play is made all the more atypical by the fact that Low and Simitian are both top-tier candidates with serious campaigns who have raised significant funds and currently hold elected office, Mitchell said.

He suggested that if there were any other recent California examples of major races where the second- and third-place finisher were this close, it would almost certainly be in a race where the dueling second- and third-place finishers were “random nobodies” who had only garnered a small segment of the vote and not serious contenders.

Eshoo announced her retirement in November after more than three decades in Congress. The district includes part of Santa Clara and San Mateo counties, including the cities of Palo Alto and Mountain View and part of the city of San Jose. Democrats hold a more than 3-to-1 registration advantage over Republicans in the district, according to data from California Target Book.

The vast majority of ballots have already been counted, but the next tranche of results from both Santa Clara and San Mateo counties is expected to be released late Tuesday afternoon.