Updated

California Lt. Gov. Gavin Newsom is endorsing his hand-picked successor for the mayor's office in San Francisco.

Newsom announced his support for interim Mayor Ed Lee during a tour of a high-tech company on Monday.

Lee, who became the city's first Asian-American mayor, is the front runner for the Nov. 8 election in all polls. Still, the endorsement likely comes as good news, as the beleaguered Lee has been facing allegations against him by his opponents, from ballot tampering to suspect campaign donations.

Lee was chosen interim mayor when Newsom was elected lieutenant governor in January.

A former city administrator who has never run for public office, Lee initially said he would not seek the mayoral seat, but changed his mind in August after a groundswell of encouragement from former mayors and the Asian-American community.

THIS IS A BREAKING NEWS UPDATE. Check back soon for further information. AP's earlier story is below.

Candidates in San Francisco's mayoral race are teaming up against Mayor Ed Lee, calling for federal and state monitors for the Nov. 8 election after a neighborhood group that supports him was accused of ballot tampering.

With early voting already under way, the seven candidates based their concerns on local media reports claiming that members of the group have been helping Cantonese-speaking residents fill out ballots.

The candidates wrote a letter to the U.S. Department of Justice and California secretary of state on Sunday, calling for an investigation into reports that the alliance used stencils that prevented voters from marking their ballots for candidates other than Lee.

"From alleged money laundering to suspected ballot tampering, those allied with Ed Lee's mayoral campaign have shown a willingness to do just about anything to preserve their power and influence inside City Hall," said Joanna Rees, a businesswoman and candidate who signed the letter.

Lee insists his campaign is not affiliated with the group, SF Neighbor Alliance for Ed Lee for Mayor 2011, and is furious over the unsolicited campaign help by the independent expenditure committee.

"I think it's moronic for them to be doing things that would interfere with people or individuals' right to vote, and if they compromise anyone they're not doing me any good at all," Lee told a morning TV show on Monday. "I have nothing to do with them."

The district attorney already is investigating donations made to the Lee campaign by some drivers of an airport shuttle company.

Lee said he welcomes the district attorney investigations into the suspect donations and ballot tampering. "In fact, I'm almost suspicious that maybe some of the other candidates have put them up to it," Lee said with a laugh.

The letter calls for federal observers and state election monitors, claiming that testimony and video evidence showed that SF Neighbor Alliance workers collected completed ballots in plastic bags and interfered with the voting secrecy.

If the ballot tampering allegations prove true, the letter states, the conduct may have violated the U.S. Voting Rights Act of 1965.

The interim mayor, who took office in January when former Mayor Gavin Newsom left office early after becoming lieutenant general, has come out far ahead of the 15 other candidates in the polls.

Although Election Day is Nov. 8, voters can cast ballots now at some designated polling stations and by mail.

The other candidates who signed the letter include City Attorney Dennis Herrera, Board of Supervisors' President David Chiu, Public Defender Jeff Adachi, City Supervisor John Avalos, State Senator Leland Yee and businesswoman Michela Alioto-Pier.

Lee's office said it believed the SF Neighbor Alliance was headed up by San Francisco attorney and political consultant Enrique Pearce. A call to Pearce's law office was not returned.

Adachi said the group is comprised of individual backers who also hand-delivered 50,000 copies of an unofficial Ed Lee biography to voters' doorsteps. He claimed the money backing their ventures came largely from two construction companies that had received millions of dollars in city contracts since 1997.

"It appears that a very tight-knit group of affluent and politically motivated individuals are at work," Adachi said in a statement calling for an investigation into the connection between Lee and the group.

"Each vote represents the voice of one citizen," Adachi said. "To cast a vote for someone else is to silence his or her voice, and to allow a few to speak for the many. It contradicts the notion of democracy, and it shouldn't happen."