ALBANY – A Democrat from Brooklyn who was elected to the State Senate last week broke ranks with his own party on Tuesday evening and will caucus with Senate Republicans come January – putting the GOP one vote away from retaining its majority in the 63-member chamber.

Simcha Felder, who represents a largely Orthodox Jewish section of Brooklyn, said he will be able to “best serve’’ his right-leaning constituents by joining with the Republicans.

The party break by Felder means the Democrats, based only on unofficial results from last week, hold a narrow 32-31 lead over Republicans to control the Senate. But there are two Senate elections where results are not yet final and it could take weeks of counting thousands of paper ballots before winners are known.

The GOP not hopeful in one of those unresolved contests, but both sides are spinning wildly about their chances in a new seat created along the Mohawk and Hudson rivers from Montgomery to Ulster counties.

If the GOP wins that race – and the Republican candidate is behind by just 139 votes with at least 8,000 absentee ballots to be counted – the Republicans will retain outright control by a 32-31 margin. Counting of those votes can’t begin until next week.

But if the GOP loses that race between two Albany-area residents, it will need at least one or more members from a group of breakaway Democrats to help create some sort of coalition-type power-sharing deal to keep the Republicans at least partly in control of the Senate. Those independent members – one each from Staten Island, the Bronx, Rockland and Madison counties – have yet to definitively state if they will rejoin their Democratic colleagues or help the GOP control the Senate if it comes to that.

The group’s leader, Sen. Jeff Klein, said Monday that his four-member caucus will remain a permanent separate group no matter how the leadership issue is resolved. The group was not commenting Tuesday night.

Senate Majority Leader Dean Skelos, a Long Island Republican, met with Felder Tuesday to broker the party-jumping deal, though Felder had already indicated before Election Day he might join with the Republicans. Skelos said the GOP’s policy priorities better match with the needs of the incoming senator from Brooklyn. It is uncertain what committee assignments or committee chairmanships Felder might get for jumping ship from his Democratic colleagues.

Mike Murphy, a Senate Democratic spokesman, said voters sent a “clear message’’ that they want Democrats to control the Senate. He said Democrats are still “confident’’ they will be in the majority come January.