Despite New York’s Democratic tilt, Republicans have controlled the State Senate for all but two years in the last century. But the threat of a “blue wave” nationally and a spate of Republican retirements has put that majority in dire risk.

“The funding class are investors, and they want to invest in something where they would see a return,” said Chapin Fay, a Republican strategist. Some Republicans had viewed Mr. Cuomo as potentially vulnerable, but his dominant win against Ms. Nixon in the Democratic primary mostly dashed those hopes, he said. “Any path to a statewide victory evaporated for Republicans on that day,” he said.

“The balance of power in the State Senate is really up for grabs this year,” added Mr. Fay, a former press secretary to the last Republican governor in New York, George E. Pataki. “That is where the money is flowing, where it can really make a difference.”

A spokesman for the Senate Republicans, Scott Reif, said they had raised $500,000 just since last Monday — which is more than Mr. Molinaro has raised since mid-July — and had more than $3 million heading into the final month, across two party accounts.

“The case we’re making is that checks and balances are important,” Mr. Reif said.

Of course, implicit in that is the idea that Mr. Cuomo will be re-elected to a third term in November.