Long the treasurer for his brother’s political campaigns, Marshall Miller has recently taken on the added role of Web site blogger.

He signs all of the campaign checks, and he regularly posts entries about new endorsements that the speaker of the City Council, Gifford Miller, receives in his quest for the Democratic nomination to take on Mayor Bloomberg in November’s election.

At the end of the month, however, Marshall, who at 34 is about a year-and-half younger than Gifford, will leave his three-year faculty post at New York University’s law school after just two years so he can join the campaign full-time.

In doing so, he will become the first family member in this mayoral campaign season to join a candidate’s payroll and to play an official role in daily campaign strategy sessions and decision-making.

“Growing up we were sort of best friends, and rivals, and brothers, all rolled into one,” the younger Mr. Miller said this week during an interview in the park adjacent to City Hall.

Marshall Miller is a softer-looking version of his brother and has an easy, mild-mannered style. Like his brother, he is slightly graying and has glassy blue eyes.

After graduating from the Massachusetts boarding school Middlesex one year apart, both brothers went on to Ivy League schools – Gifford to Princeton and Marshall to Yale, where he also completed his law degree.

Now, as Gifford moves at a feverish speed among council events, news conferences, community forums, and parades, usually in his government uniform of white button-down shirt and navy suit, his brother works in the more relaxed campaign office on the seventh floor of a low-rise building on Nassau Street around the corner from City Hall.

Marshall Miller, who worked as an assistant U.S. attorney for the Eastern District of New York before taking the position at NYU, said he has worked on several political campaigns, including one with Gifford for Rep. Carolyn Maloney, Democrat of the Upper East Side.

He will retain the title of treasurer but will also be responsible for handling legal work for the campaign and making sure donations and expenditures comply with regulations of the city’s Campaign Finance Board. He will continue blogging and will pitch in with research for the candidate’s policy speeches.

“He’s one of the smartest and most talented and hardworking people I’ve ever met, so it’s great to have those abilities on the campaign, but it’s also fun to be in the trenches with your brother,” Gifford Miller said during a phone interview.

Marshall Miller is not the only one in the Miller clan working to get the speaker – who has raised more money than his three Democratic opponents but still trails them in the polls – the party nomination.

Gifford Miller’s wife, Pamela Miller, is already volunteering for the campaign part-time and plans to take a leave soon from her law firm, Arnold & Porter, to go full-time with the campaign – though not as a paid employee.

And the speaker’s sister-in-law, Eileen Miller, a graduate of the University of Pennsylvania’s Wharton business school, is doing administrative work and bookkeeping for the campaign three days a week, also as a volunteer.

“It’s not every day that your brother runs for mayor, or your son, or brother-in-law,” Marshall Miller said.

The campaign would not disclose how much Marshall Miller would be paid but will have to disclose his salary in future filings with the city’s Campaign Finance Board.

Family support on the campaign trail is nothing new and will probably become more visible in the coming months.

Rep. Anthony Weiner, another of the Democratic mayoral candidates, said yesterday that his parents have played significant roles in his campaign so far. His mother, a retired high school teacher, is his education guru and, according to Mr. Weiner, reminds the campaign staff to “feed me once in a while.”

The congressman, whose district crosses the Brooklyn-Queens border, refers to his father as a walking “Op-Ed page,” with opinions on nearly every issue that comes up during the campaign.

Though Messrs. Weiner and Miller disagree on many policy issues, both said the proximity of family helps keep things in perspective during the whirlwind of a campaign.

“It’s an enormous undertaking, and having the people you love and trust most is critical for your emotional well-being,” Mr. Miller said.

The Democratic front-runner, Fernando Ferrer, a former borough president, does not have any family members on staff, but a campaign spokeswoman, Jennifer Bluestein, said his wife, Aramina, an elementary-school principal, will take on a more visible role soon.

The borough president of Manhattan, C. Virginia Fields, has most of her family in Alabama, where she grew up – but she talks to them regularly about the mayoral race, her staff said.