Sen. Simcha Felder may be on the political hot seat from now till November.
Democrats have prevailed in two special elections for the NY state Senate but Republicans will retain control thanks to the support of a breakaway Democrat.
"I believe it is my obligation to prevent an unprecedented and uncertain late session political battle that will only hurt my constituents and New Yorkers", he said.
There were also nine seats at stake in the state Assembly in the special election.
Those results will leave the chamber split 31-31, with Felder holding the deciding vote.
Simcha Felder has long sat with Senate Republicans, to the frustration of other Democrats. Even though the last time a Democrat occupied the seat was four decades ago, Democrats outnumber Republicans in the district by almost 6,000 registered voters.
Mr. Felder, however, apparently unswayed by the governor's previous entreaties, said he had to do what was best for his constituents, which includes a large community of Orthodox Jews, though he stressed he had "great respect for my colleagues on both sides of the aisle and this institution". His power - and the pitfalls that it entails - came into focus when he singlehandedly held up the state budget to extract concessions around the way the state regulates Orthodox yeshivas.
"Even the reddest of districts in a red state can be in play this year", the Associated Press wrote in an article analyzing Tuesday's slim win for the GOP in Arizona.
"I'm asking him to think about the critical place that we are", said Senator Andrew Stewart Cousins (D-Yonkers).More news: Dow opens slightly higher on industrial gains
But the "Trump effect" is not only helping Democrats in local races. On Wednesday, Felder said he had not yet read Cuomo's letter.
In addition to the Westchester race, there is another Senate race in the Bronx and nine Assembly races in districts from Long Island to Buffalo.
But Democratic officials in NY saw the outcomes very differently.
A Democratic victory in the election Tuesday would set up a new fight in the legislature's upper chamber as the party would then likely hold a one-member numerical majority over Republicans.
Republicans now have a one-seat Senate majority.
Republicans, meanwhile, said they expected Felder to follow through on his promise and remain with the GOP.
"The governor's position is clear: the Democrats must unify to take back the majority", Lever said.