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

Dan Janison has been a reporter at Newsday since 1997.

Twenty-one months can mark a long time in politics.

Right now, Sen. Charles Schumer becomes a leader in waiting, widely expected to win the top Democratic post in the nation's upper house by the end of 2016 when Senate Minority Leader Harry Reid (D-Nev.) retires.

Currently No. 3 in the party pecking order, Schumer has worked toward this for years, raising millions of dollars and donating it to colleagues, surpassing the funding performance of the No. 2, party Whip Dick Durbin of Illinois.

Reid on Friday endorsed Schumer to succeed him as he revealed he won't seek another term. Durbin, too, got behind the third-term former congressman and state legislator from Brooklyn. The path to the prize looks clear as can be.

And yet, Schumer would need to maintain support among colleagues, ensuring a rival doesn't chip away at it -- and also win re-election next year. Although both Daniel Patrick Moynihan and Jacob Javits served four terms in the Senate, "three-and-out" has been a general rule for big New York pols in recent decades.

Given his cordial ties with Wall Street domestically, and with the right-leaning government of Israel on the foreign front, could Schumer face a substantive primary challenge from his left?

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Might voices like those of fellow Democrat Elizabeth Warren and Independent Bernie Sanders turn to negative static for Schumer?

Uncontrolled events, and how Schumer reacts to them, could shape the answer.

Might Republicans -- perhaps cashing in on post-Barack Obama tensions within the Democratic ranks -- find a way to threaten his incumbency in New York?

Mainstream Democrats don't seem to think so, given that naming his 2004 and 2010 opponents could make for a tough trivia question (Answer: Howard Mills and Jay Townsend). They expect the national GOP to invest in less dicey races.

On Friday, though, the incorrigibly optimistic Ed Cox, state Republican chairman, reacted to news of Reid's endorsing Schumer with a statement titled "Not So Fast." He said, "New York is still the highest-taxed, worst state in America to do business, and next year, New Yorkers will choose the substance we sorely need over more grandstanding."

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Much will be made of the prospect of his former junior senator, Hillary Clinton, topping next year's ballot with Schumer positioned to head her party's Senate caucus.

Still, for both, 21 months can mark a long time in politics.