Police Commissioner William Bratton is taking on a familiar role in his fight to boost the NYPD's head count: disagreeing with his boss.

His clashes as commissioner two decades ago with then-Mayor Rudy Giuliani led to his early exit from the job. Political leaders in Los Angeles were more tolerant when Bratton spoke his mind -- and took opposing policy stands -- during his seven years as LAPD chief.

It's now Mayor Bill de Blasio's turn. On Thursday, Bratton is set to testify before a largely like-minded City Council to make a case to add hundreds of cops to the 35,078-member NYPD. The council has proposed an increase of 1,000 officers.

In de Blasio's executive budget plan May 7, there was no change. By then, Bratton had already made his case public -- and in a more direct manner than in the Giuliani days.

"It is unusual, at least in my memory, for a police commissioner to so publicly lobby his boss and be in conflict with his boss," said Baruch College political scientist Douglas Muzzio.

Bratton has also been negotiating in private with de Blasio and city budget officials.

Confirming those talks were continuing, Bratton said Wednesday: "There is a favorite movie of mine about Leonardo da Vinci painting the Sistine Chapel and the pope coming back from fighting the wars [and asking]: 'When will it be done?' Well I sometimes feel like the pope asking 'When will it be done?' " (The artist was actually Michelangelo.)

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A high-ranking law enforcement official cautioned about expecting Bratton to ask for a specific number of additional cops, but said Bratton would repeat how the changing face of terrorism has created additional NYPD needs. Bratton previously has said he needs at least 400 more to guard against the evolving terror threat since the rise of the Islamic State group.

De Blasio, asked about the NYPD head count as he unveiled his draft executive budget, said he's "very confident in what's happening right now with the resources we have."

Hours later, Bratton's spokesman emailed reporters: "The police commissioner is confident that there will be an increase in the size of the force."

By law, the budget must be finished by the end of June.

The dispute between Bratton and de Blasio has remained polite, said Thomas Reppetto, former head of the city's Citizens Crime Commission.

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"Bratton is not any commissioner. He was brought here to be a star," Reppetto said.

After de Blasio was elected, in part on promises to curb controversial police practices such as stop-and-frisk, his choice of Bratton was seen largely as a move to reassure New Yorkers who feared he would be a "soft-on-crime mayor," Muzzio said.

"Bratton gave him immediate legitimacy and continues to do so, because Bratton comes with a substantial police reputation, and to lose that would be very damaging," Muzzio said.

They have stood together with no sign of daylight between them as the NYPD has been buffeted by protests over the Eric Garner accused chokehold case, complaints from minority and civil-rights groups about Bratton's "broken windows" crime-fighting beliefs, and police unions accusing de Blasio of not having their backs.

But on staffing, Bratton has chosen to leverage his stature.

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"When the police commissioner says something, he is listened to," said political consultant Hank Sheinkopf.

At 67, Bratton has his share of political battle scars.

Disputes with Giuliani over personnel, budgets -- and who got to bask in the spotlight -- led to Bratton's ouster in 1996.

As LAPD chief during the next decade, Bratton sought authority over policing at the city's international airport. Mayor Antonio Villaraigosa opposed him, but a compromise was found. Bratton left Los Angeles widely praised for cleaning up a scandal-scarred force.

When he accepted de Blasio's offer in December 2013, Bratton promised he had "learned a lot" since his Giuliani days. De Blasio said he'd done due diligence to make sure Bratton had.

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"I report to the mayor," Bratton said then. "I am not the mayor."