"I continue to reflect on three things that are important to me: my wife, my daughter and my commitment to the city," he said.
Newsom cited those same reasons for dropping out of the governor's contest after a year-long campaign.
But what is a 42-year-old to do? The two-term mayor will be termed out of office in November 2011.
"Those days are numbered and to the extent that I can still offer something, and I believe I can; I hate to just end it, I feel like I can still contribute," Newsom said.
Newsom had initially dismissed the idea of lieutenant governor, calling it a largely ceremonial post. Then a statewide survey made him reconsider.
The poll by pollster Ben Tulchin shows Newsom with 33 percent of the vote, way out in front of the two announced candidates for the Democratic primary, Los Angeles city councilwoman Janice Hahn and state Sen. Dean Florez from Kern County.
"He does well, not just in the Bay Area, but in the central valley and other parts of the state, so that gives him a good base on which to run," Tulchin said.
Candidate statements for the voter guide are due Wednesday. It is not a requirement, but could signal Newsom's intent.
If one takes this guessing game to the finish line, and the mayor were to win the Democratic primary, then beat the Republican candidate and win in November, he would step down as the city's top politician. Under the city charter, the president of the Board of Supervisors, currently David Chiu, would take over for a short time. Then the entire Board would appoint a successor. According to Chiu, that person could be any registered voter in the city. Newsom joked Tuesday that former Mayor Willie Brown keeps encouraging him to run for lieutenant governor because, "I don't know if that's because he wants to be mayor."
The guessing game officially ends March 12, the deadline for entering the primary.