Dan Janison Melville. N.Y. Tuesday January 26, 2010. Daniel Janison,

Dan Janison has been a reporter at Newsday since 1997.

Some of Bill de Blasio's detractors would like to believe he possesses the right qualities to make him New York City's first one-term mayor in a generation.

Pundits drive this one-term meme by citing controversies over police issues, charter schools, the clashes with Gov. Andrew M. Cuomo, Uber taxis and the mayor's chronic lateness and out-of-town travel.

His poll ratings have sagged, of course. But don't bet the Metro Card just yet against de Blasio in 2017. Nineteen months into the Brooklyn Democrat's first term, there's no sign of a strong challenger laying groundwork inside or outside de Blasio's dominant party.

Yes, it remains early. But, the "one-term" speculation often ignores a big factor -- the unseen hand of the city's two-term limit in all municipal offices.

The certainty that an incumbent will leave after the next term seems to make it easier for him to win that one re-election.

Term limits have shaped the field of every municipal race since their 1993 enactment. That's because potential challengers may want to save their energy and funding for when the job automatically opens on its own.

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"Be patient, he's gone in four years," could become an opposition mantra.

The field of likely Democratic mayoral contenders usually includes secondary city officials. Comptroller Scott Stringer and Public Advocate Letitia James came in with de Blasio in January 2014. They'd have to quit those posts to take a whack at de Blasio in 2017. But in 2021, the mayor's job opens up just when they're leaving.

City Council Speaker Melissa Mark-Viverito (D-Bronx) also ascended to her powerful post last year. But she must leave the Council in 2017, having served three terms under a bizarre special bill that allowed Mayor Michael Bloomberg a third term. While free to run, Mark-Viverito is generally seen as a de Blasio ally unlikely to run against him.

More likely, perhaps, a primary comes from a candidate such as Rep. Hakeem Jeffries (D-Brooklyn), who need not quit his day job to try.

The last two mayors, elected under term limits, cruised to second terms. But Hunter College political science professor Ken Sherrill sees offsets to the mayor's term-limit advantage.

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The city's public campaign finance system "might enable a vigorous campaign by a breakaway Democrat" on the Republican line, he said, "perhaps with the governor's tacit approval." Even then, Sherrill said, de Blasio "probably would have a get-out-the-vote advantage from unions."

"In any case, de Blasio is likely to be weakened in the second term by virtue of term limits, by the ambitions of people running for 2021, and what's likely to be a narrower margin of victory in 2017," Sherrill said.

In Congress and the State Legislature, which don't have term limits, incentive works the other way. Rookie lawmakers, with two-year terms, are viewed as most vulnerable, having not been around long enough to secure their incumbencies.

The city's last one-term mayor was David Dinkins. In 1989, before term limits, he beat Democratic Mayor Ed Koch in a primary and former prosecutor Rudy Giuliani in the general election. Republican Giuliani unseated Dinkins four years later, beginning 20 years of GOP mayors.

Republican strength in the city has waned, making all the more poignant a recent bumper sticker that says, "Don't Blame Me -- I Voted for Joe Lhota," the Republican de Blasio beat in a landslide.