Rudolph W. Giuliani, already under criminal indictment and at risk of losing his law license for his effort to keep Donald J. Trump in office after the 2020 election, is now being sued by his own lawyer.

The lawyer, Robert J. Costello, who had been leading Mr. Giuliani’s defense against an onslaught of legal woes, signed onto the lawsuit brought by his firm on Monday to recover more than $1.3 million in unpaid legal fees. The development deals a stunning blow to Mr. Giuliani, as he nears a financial breaking point.

Mr. Giuliani, the former New York City mayor, ex-U.S. attorney and onetime high-flying litigator, owes nearly $3 million to various law firms, including the one where Mr. Costello is a partner, people with knowledge of the matter have said. Mr. Giuliani had repeatedly sought a financial lifeline from his client, Mr. Trump, who had offered vague promises to pay up but had largely refused to do so.

Much of what Mr. Giuliani owes arose from his work for Mr. Trump in the aftermath of the 2020 election. Mr. Giuliani racked up legal bills while battling an array of criminal and congressional investigations, private lawsuits and disciplinary proceedings that cast a harsh light on his bid to keep Mr. Trump in office in spite of his election loss.

The lawsuit from Mr. Costello and his firm, Davidoff Hutcher & Citron LLP, represents a new low for Mr. Giuliani and, in his eyes, something of a personal betrayal.

The two men have known each other for a half century; Mr. Costello trained as a law student under Mr. Giuliani, who at the time was a federal prosecutor in Manhattan. They became friends and, for the last four years, Mr. Costello was Mr. Giuliani’s loyal and pugilistic defender against a drumbeat of legal scrutiny.

In a statement provided by a spokesman, Mr. Giuliani lashed out at Mr. Costello and the lawsuit, portraying it as an overly aggressive attempt to collect.

“I can’t express how personally hurt I am by what Bob Costello has done,” Mr. Giuliani said. “It’s a real shame when lawyers do things like this, and all I will say is that their bill is way in excess to anything approaching legitimate fees.”

Reached by phone, Mr. Costello initially declined to comment but fired back after hearing Mr. Giuliani’s statement, asking, “How can he take a personal affront when he owes my firm nearly $1.4 million?”

Mr. Costello also disputed the claim that the bills were excessive, saying that he billed his regular hourly rate and that Mr. Giuliani never complained about the cost until Davidoff Hutcher & Citron warned that it had planned to sue.

“He’s a little late to that party,” Mr. Costello said, adding, “it’s too late for that frivolous claim as he will find out in court.”

Mr. Giuliani, he said, “took the low road here because he is feeling desperate.”

In all, Mr. Costello’s firm has billed Mr. Giuliani $1,574,196, according to the lawsuit. Of that, Mr. Giuliani has paid only $214,000, the lawsuit said, most recently handing over $10,000 last week.

The bills reflect a wide array of work, beginning in 2019, when Mr. Giuliani was under investigation by federal prosecutors in Manhattan scrutinizing his ties to Ukraine. When Mr. Giuliani’s legal problems ballooned, Mr. Costello oversaw the various tentacles of litigation arising from the 2020 election fight.

Mr. Giuliani’s efforts for Mr. Trump led to the suspension of his law license in New York. This summer, a legal ethics committee in Washington recommended that Mr. Giuliani be disbarred for his “unparalleled” attempts to overturn the election.

Two Georgia election workers also filed a defamation lawsuit against him because he falsely accused them of stealing ballots; a trial is set to be scheduled soon to assign damages.

And in August, Mr. Giuliani was indicted alongside Mr. Trump in a sweeping racketeering case brought by a prosecutor in Georgia.

Mr. Costello, who was not working pro bono, led an effort to recover money from Mr. Trump for more than two years. Mr. Costello has contacted at least six lawyers close to Mr. Trump, making the case that the former president was on the hook for legal fees that Mr. Giuliani incurred because of his work on the election.

In April, Mr. Costello and Mr. Giuliani flew to Florida to make a direct appeal to Mr. Trump, meeting him at his golf club in West Palm Beach and again at his Mar-a-Lago estate two days later.

At Mar-a-Lago, Mr. Trump offered an assurance that Mr. Giuliani’s bills would be taken care of, a person with knowledge of the matter has said, but since then the former president has yet to pay up. His political action committee, which has covered legal bills for other people in his circle, has paid only $340,000 on Mr. Giuliani’s behalf.

A fund-raiser for Mr. Giuliani was recently held at Mr. Trump’s golf club in New Jersey, and some of the former president’s aides were told that it raised $1 million.

Mr. Giuliani, whose response to the Sept. 11, 2001, attacks led to his being called “America’s mayor,” was once worth tens of millions of dollars.

He has seen a remarkable reversal of fortunes since going to work for Mr. Trump. He also lost much of his wealth in a divorce from his third wife, Judith Nathan, and is currently unable to work as an attorney because of the disciplinary actions against him.

Mr. Giuliani, whose apartment on Manhattan’s Upper East Side was searched by federal authorities in 2021 as part of the investigation into his Ukraine dealings, recently put it on sale for $6.5 million.

Maggie Haberman contributed reporting.