Cynthia Nixon is still a long ways back in the polls, but she's moving up.

Now, a month after his former top aide was convicted of bribery and facing a spirited primary challenge, Cuomo's popularity has plummeted, a Siena College poll Tuesday found.
Cuomo's favorability rating fell to 49 percent positive and 44 percent negative this month, matching his lowest rating as governor, the poll from the Albany-area college said.

Nixon may be new to politics, but it's Cuomo that is making the rookie mistakes.
The Daily Beast says "he looks rattled early on".
He appears to be trying too hard. That includes getting dirty when it isn't necessary, and in a public way.

Nice rally you have there. It’d be a shame if anything happened to it.
Gov. Andrew M. Cuomo’s aides and campaign staff made a round of phone calls to elected officials, asking them to not attend a rally on Thursday in support of community groups that have endorsed his opponent, Cynthia Nixon, according to eight New York City Council members who received such calls or messages and two others briefed on the effort...
Mr. Cuomo has denied using any such pressure tactics, even as his aides pressed various Council members to boycott the rally.
The organizers of the rally included three groups — Citizen Action, New York Communities for Change and Make the Road Action — that have ties to the Working Families Party and endorsed Ms. Nixon.
“If unions or anyone give money to any of these groups, they can lose my number,” Mr. Cuomo said in a private meeting last week, according to Bill Lipton, the state director of the Working Families Party, a progressive alliance that backed Ms. Nixon last weekend.

What a nice guy. A real Tony Soprano.

With such an enormous lead, why go to such lengths?
The reason is because of Zephyr Rain Teachout and her 2014 campaign. Cuomo literally never even said her name until election night.
Four years ago Cuomo took both Teachout and progressives for granted.

The strong showing by Teachout and Wu was a victory for progressive voters who warmed to their message about tackling rising inequality, political corruption, and corporate abuses. It was also a rejection of Cuomo's economic philosophy, which led him to introduce a series of tax cuts for the rich, at the same time that he cut the state budgets for education and social services. I'd be willing to wager that most Democrats who voted against Cuomo objected more to his policies than to his personality.

Teachout and Wu’s insurgent campaigns gave voice to this sentiment. Eschewing the etiquette of internal party discourse, Teachout accused Cuomo of governing as a Republican, acting as a shill for the big banks and other campaign contributors, and being part of a "corrupt old boys' club" in Albany. Making full use of social media and appearances in more traditional media, she demonstrated that, even in this day and age, a candidate with a real message doesn't necessarily need the support of the party apparatus, or the financial backing of big donors, to have an impact.

Sound familiar? Nixon is saying the same things today.
The difference is that it's harder to fool people the second time around.

Cuomo, a Democrat, promised to back a series of progressive policies and a Democratic takeover of the state Senate -- including the reunification of mainline Democrats and the Independent Democratic Conference -- where he acknowledged that many of those policies had been blocked.

Nearly four years later and with little he had promised the WFP accomplished, Cuomo made similar pronouncements, this time weeks ahead of the planned WFP convention, while he is facing a challenge from the left by actor and activist Cynthia Nixon
...Without acknowledging he did not keep any prior promises, Cuomo, again seeking the WFP endorsement until it became clear he would not get it, has said this time will be different.

But the relationship between Cuomo and progressive activists appears beyond repair...

After sh*tting on progressives for eight years, Cuomo has suddenly gone into overdrive trying to get on their good side. He's suddenly pushing through progressive policies that have gathered dust for 8 years.
It's unlikely to work. Cuomo campaign spokesperson Abbey Fousher claimed that “The governor and Sen. Sanders are in lock-step.” Implying that Sanders endorsed Cuomo.
Sanders senior adviser Ari Rabin-Havt gave a classic reply, “The idea that Andrew Cuomo and Bernie Sanders are lock step on policy is 100% Grade A American bullshit.”

Teachout, without name recognition, without endorsements, and without funds, still managed to get a third of the votes.
Nixon has name recognition, endorsements, and funds. And she is running in an election in which progressives are excited.
That's why Cuomo is sweating.