Rep. Robert Brady’s (D-Pa.) expected announcement today that he will join fellow Rep. Chaka Fattah (D-Pa.) as a candidate for mayor of Philadelphia creates an unusual dynamic: a mayoral primary that includes two sitting Members of Congress.

Voters in a major American city have not confronted such a choice since New Yorkers did in the 1970s. Two consecutive mayoral elections had Democratic Members running against each other for the keys to Gracie Mansion.

Are there lessons for Fattah and Brady, half of Philadelphia’s Congressional delegation, to take away from those New York primaries?

In 1973, two Bronx Congressmen, then-Rep. Mario Biaggi (D) and then-Rep. Herman Badillo (D) faced off in a four-candidate Democratic field. Then-Rep. Ed Koch (D-N.Y.) also briefly flirted with the race but dropped out after six weeks due to sluggish fundraising.

In 1977, Badillo again was pitted against a fellow Member — this time an ultimately successful Koch — in a crowded seven-person Democratic contest, which featured the incumbent mayor, Abe Beame, and a future governor, Mario Cuomo (D).

So was it tough running against a Congressional colleague?

Not at all, say these former Members.

Biaggi, who started out as the 1973 frontrunner before being caught in a lie — he denied that he had pleaded the Fifth before a grand jury investigating alleged payoffs to Members over immigration legislation, when in fact he had — couldn’t recall any bad blood between himself and Badillo.

“There was no tension,” he said. “Each of us knew long beforehand what our aspirations were.”

He added: “We had a commonality of interests. … We got along well in the Congress except that we both wanted the same thing at the time.”

Still, Biaggi admitted: “We were never really friends. … We were civil.”

Now 89, the colorful Biaggi, a decorated former police officer who would resign from Congress in the late 1980s after being convicted on federal corruption charges, said he has no regrets about his handling of the 1973 race.

“That’s life,” he said of being caught in his own lies. “I was yards ahead of everybody. I was the overwhelming favorite as far as the odds went. … The roof fell in.”

Badillo, 77, who is now of counsel to a New York law firm and a senior fellow at the Manhattan Institute of Policy Research, ultimately would make it to the Democratic runoff in 1973, only to lose to Beame, who was then New York’s long-serving city comptroller.

Fellow members of the New York Congressional delegation were generally careful to refrain from taking sides, Biaggi said.

“They didn’t want to get in between us because they knew it would be over soon and they would have to live with each other,” he said.

“We were all Democrats, there were no problems,” said Badillo, who after several unsuccessful tries to become mayor as a Democrat would switch to the Republican Party and run again in 2001, losing the GOP primary to now-Mayor Michael Bloomberg (R).

Koch agreed, but he couldn’t resist a little dig. He said he and Badillo were able to remain friendly in that crowded contest because “Herman was not one of the leading contenders,” he said. “My race was basically against Mario Cuomo.”

Koch said that it was a former Member of Congress, ex-Rep. Bella Abzug (D-N.Y.), who gave him some of his biggest headaches when they were running against each other during the crowded 1977 primary.

“We were never the best of buddies,” he recalled, pointing to her denunciations of him during the race — likely retaliation, Koch said, for his support for her opponent, then-Rep. Bill Ryan, in a 1972 Democratic Congressional primary. (Koch told newspaper reporters that Abzug lost to Ryan because “her neighbors know her.” Ryan died shortly after his primary with Abzug, and she replaced him in Congress after serving a term in a different district.)

But all that’s water under the bridge, Koch, who is now 82, asserted. Later in life, purely coincidentally, he moved into the same Greenwich Village apartment building where Abzug was living. She died in 1998 at the age of 77.

“I advocate that we build a statue to [Abzug] in Washington Square Park,” he said.

Meanwhile, any hard feelings between Badillo and Koch didn’t start until after their Congressional careers. Shortly after he became mayor, Koch appointed Badillo one of his deputy mayors, but they eventually had a falling out and Badillo resigned.

They’ve since patched up any differences and occasionally meet for dinner. Koch even wrote a blurb for Badillo’s recent book, “One Nation, One Standard: An Ex-Liberal on How Hispanics Can Succeed Just Like Other Immigrant Groups.”

Did any of these men have any advice for Fattah and Brady?

“No dirty tricks,” Koch said.

Meanwhile, Biaggi, who attempted a Congressional comeback in 1992 and currently is writing his memoirs, said it was important to remain philosophical.

“Disagree if you want, but don’t be disagreeable,” he said. “You had a life before you ran for mayor and you’ll have a life after it.”