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WASHINGTON — Cynthia Nixon has a new role.

The "Sex and the City" actress announced Monday that she has decided to challenge Gov. Andrew Cuomo, who is seeking re-election to a third term, in the Sept. 13 Democratic primary.

Nixon, an activist and performer best known for playing Miranda Hobbes on the hit HBO show and in several movies, is the clear underdog, but Cuomo has made plenty of enemies and may have a real fight on his left flank.

"I love New York. I’ve never lived anywhere else," Nixon said in her announcement video. "But something has to change. ... We are sick of politicians who care more about headlines and power than they do about us. It can't just be business as usual anymore."

Her announcement also called attention to allegations of corruption (one of Cuomo's former top aides was just convicted on pay-to-play charges last week), neglect of the New York City subway system, which the governor has a big hand in, and shady deals with Republicans in Albany to give them control of the state Senate.

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Anticipating the challenge, Cuomo has been flexing his political muscle and firing warning shots, especially at his bete noire, New York City Mayor Bill de Blasio.

Nixon is a prominent supporter of de Blasio, and several of his top aides are involved in her campaign, leading some Cuomo allies to see Nixon as a proxy for the mayor.

Beyond show business, Nixon has been active in New York City politics, especially around public education and same-sex marriage.

Cuomo is in a strong position politically, with 66 percent of registered Democrats supporting him, compared with 19 percent for Nixon, according to a recent Siena College poll. But Nixon has room to grow, with around six-in-10 Democrats remaining undecided about her.

"Nixon knows the hill is a very steep one, especially given Cuomo's massive money advantage, but Cuomo actually also has a herculean task before him," said Jonathan Tasini, a progressive activist who challenged Hillary Clinton’s re-election as senator from New York in 2006, just before she launched her first presidential campaign.

"If (Cuomo) is seriously considering running for president in 2020, he can't have Nixon run up a north-of-40 percent number, which is entirely doable because the energized progressive wing has no big love for the incumbent," Tasini said.

Liberal activist Zephyr Teachout, who won nearly 35 percent of the vote against Cuomo in the 2014 Democratic primary, is Nixon's campaign treasurer.

Cuomo appears to have been taking steps to brace himself for the challenge, running TV ads and rolling out big endorsements, even as he dismisses Nixon as a B-list celebrity.

Last week, the governor made a show of laying on the ground in Zuccotti Park, the erstwhile home of Occupy Wall Street, for a "die in" to protest gun violence.

And he's been stepping up his attacks on de Blasio, using a speech at a church Sunday to call the city’s public housing "disgusting" and to compare the city's Rikers Island jail to South Africa's notorious Robben Island, where Nelson Mandela was imprisoned.

Cuomo invoked his father, former three-term New York Gov. Mario Cuomo, and said De Blasio's 10-year-plan to rebuild the prison is too slow.

"We're building a new LaGuardia Airport, it takes four years. We're building a new bridge over the Hudson River, it takes five years. They wanted to build Yankee Stadium, it took them one year," Cuomo said of de Blasio's lengthy plan to rebuild Rikers, according to the New York Daily News. "You know what it means when they say it takes them 10 years to build a jail? It's not a priority."