Democratic mayoral nominee Bill de Blasio tried to paint Joe Lhota's Republicanism on his chest like a scarlet letter "R" Tuesday night in their first debate.

De Blasio, with a commanding 3-1 lead in the polls, repeatedly hammered Lhota as a "mainstream Republican" taking pages "right out of the Republican playbook."

"We don't need Republican trickle-down economics or tea-party extremism," de Blasio said. "We need a new, progressive vision for this city that leaves no one behind."

Lhota said de Blasio was "untested" as a big-city executive. He contended the Democrat's policies on police, taxes and education would send the city spiraling downward.

"I've been there before and I've done it. I can be there on day one without any training, without any learning curve whatsoever," said Lhota, who was deputy mayor and budget director in the Giuliani administration.

De Blasio, the city's public advocate, sought to turn Lhota's resume against him.

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"Yes, Mr. Lhota was a top aide to Rudolph Giuliani, the most divisive administration we've seen in decades," he said.

For much of the hour, Lhota was on the defensive, struggling as he has for weeks to distance himself from the national GOP and its role in the federal government shutdown.

"Don't lump me in with people I'm constantly in disagreement with," Lhota said.

Speaking to reporters after the debate, de Blasio stood by the line of attack. "The company you keep matters here," he said.

Lhota hit back harder than he had while on TV, saying de Blasio continues "to lie about who I am and what I am." He called his opponent "your typical, classic, political hack who doesn't know what to do when he's in a debate and talking about issues."

Lhota has been banking on the debates to reverse polls showing de Blasio in reach of a landslide win. But Lhota's performance was unlikely to move the needle, and de Blasio dominated, said Christina Greer, a Fordham University assistant professor of political science.

"He was on the offensive, and Lhota was completely on the defensive, and when you're 50 points down, you have to be on the offensive," Greer said.

She added: "He sort of got lost in the weeds, and there were so many opportunities for him to really distance himself from de Blasio and make some key points, but he never did."

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Two more debates are set for Oct. 22 and 29.

The mayoral rivals disagreed on most issues the panel at WABC-TV put to them:

On charter schools in public school buildings, de Blasio wants to charge many of them rent; Lhota considers them a much-needed alternative for students at failing schools.

On business incentives, de Blasio said Lhota "never met a corporate subsidy he didn't like." Lhota called them vital to create and preserve jobs.

On police practices, de Blasio said, "We have to end the stop-and-frisk era that has so unfairly targeted people of color." Lhota said the NYPD is carrying out the policy constitutionally and that it's necessary to keep crime down.

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Lhota said voters shouldn't trust de Blasio's promise to limit tax increases only to the wealthiest New Yorkers. "Believe me when I say this," Lhota said. "He will not stop."

De Blasio retorted, "I talk about one tax and one tax only," he said.

Even when they both agreed on a problem -- for instance, that housing prices in New York City are skyrocketing into unaffordably -- Lhota disputed the legality of some pressures de Blasio would use to require developers to include more affordable housing below market rates.