The race in a newly created state Assembly district that encompasses part of Sonoma County and all of Marin County is starting to heat up in one of the few contests statewide to feature two Democrats running against each other.

Assemblyman Michael Allen and his challenger, Marc Levine, earned the right to compete for the 10th Assembly District under California's new top-two primary system.

The race has gained attention in part because it is one of about two dozen in the state pitting members of the same party against one another in the general election on Nov. 6.

Allen, a former union leader who has risen quickly through the ranks of power in his first Assembly term to become assistant majority floor leader, has the support of Democratic Party leaders in Sacramento.

The only hope that Levine, a first-term San Rafael city councilman, has to counter that support is to run an "insurgent candidacy," said David McCuan, a political scientist at Sonoma State University.

McCuan said Levine faces a "significantly uphill battle because the powers of Sacramento, and the interest groups around Sacramento, do not want to see a fractious Democrat-on-Democrat battle."

Allen did not return messages left over three days this week seeking comment on his candidacy.

He and Levine have gotten testy with one another over everything from endorsements to Allen's decision to relocate to San Rafael in order to qualify to run in the redrawn district.

On Tuesday, a campaign aide to Levine alleged in a letter to Sonoma County tax officials that Allen violated state law by claiming a homeowner's taxation exemption that Allen is no longer entitled to after his move to Marin.

The complaint was filed by Bruce Raful, Levine's campaign treasurer and a certified real estate appraiser.

"The California Constitution is pretty clear," Raful said in a statement sent to reporters. "If you live in your home, and don't rent it out or vacate it, you're entitled to a $7,000 tax exemption.

In fact, the Constitution does not provide for an exemption of $7,000 in taxes. It does exempt $7,000 in assessed value from application of the property tax rates.

"Mr. Allen left Oakmont for San Rafael in October of last year when he re-registered there to vote, so he wasn't entitled to the exemption that he claimed on January 1 of 2012. In my book, that's cheating," Raful wrote.

Sonoma County Assessor Janice Atkinson on Wednesday said Raful is correct that Allen should not have claimed the exemption. But she called it an "oversight" on Allen's part.

Atkinson said such exemptions are claimed once and then automatically renew each year. She said it's not uncommon for property owners to forget to notify tax officials when they move so that they no longer get the benefit.

Atkinson said for a piece of property valued at $300,000, the exemption amounts to a property tax savings of $70. She said Allen will have to pay back his exemption savings for 2012 plus 25 percent of that amount as a penalty.

Atkinson said Allen visited the assessor's office Wednesday to correct his paperwork.

Levine's campaign also this week released results of a poll that appears to show he is leading Allen in the race.

The poll, conducted of 400 likely voters in Sonoma and Marin counties by Global Strategy Group, found 26 percent supported Levine and 19 percent backing Allen.

McCuan said the sample size was too small and the poll too aligned with the Levine campaign to draw any firm conclusions. He said people are just now tuning into the race.

McCuan predicted the race could get ugly in coming weeks as the candidates and the groups that support them unleash what McCuan expects will be a deluge of negative ads.

At the May deadline for campaign finance reporting, Allen had raised seven times more than Levine, mostly through donations from the Democratic State Central Committee and labor groups.

The next reporting deadline is Oct. 5.

(You can reach Staff Writer Derek Moore at 521-5336 or derek.moore@pressdemocrat.com. On Twitter @deadlinederek.)