Barney Frank at the Democratic National Convention in September. (J. Scott Applewhite/AP)

Barney Frank, finally a senator!

The former congressman — who is currently angling for John Kerry’s Senate seat — will pretend to be one Saturday when he appears in a New York City Center production of “Fiorello!” Frank will play a cranky conservative senator who tries to rein in upstart Fiorello LaGuardia.

“I’m playing against type,” Frank told us Monday.

Three weeks into retirement, the liberal pol is having all sorts of fun. Last week, he made his first paid speech. How much? “Couple months pay as a congressman,” he said.

And now, a role in his favorite musical.

Frank saw the original production of “Fiorello!” in the late ’50s (he was able to recite lyrics to two songs off the top of his head for us) and jumped at the chance when artistic director Jack Viertel asked him to do the one-night cameo. The part has about eight lines but, sadly, no singing.

“It’s really just one scene,” said director Gary Griffin. “But it’s important — it sets the plot in motion.”

Frank, of course, has his eye on another role these days: Kerry’s Senate seat. He broke political convention by announcing that he’d like the appointment because Congress will be voting on important financial reforms he worked on last year.

If Frank does get the gig, it will be but a cameo as well, lasting only until a June special election. Kerry is expected to be confirmed as secretary of state this week, after which Massachusetts Gov. Deval Patrick will announce his replacement.

“The governor correctly is not saying or doing anything until John Kerry is confirmed,” Frank told us.

But, just between us, can we call you “Sen. Frank”?

“At least someone will,” he said with a laugh, “on Saturday.”

Earlier: Deval Patrick, Barney Frank and the politics of Senate appointments, 1/10/13

Barney Frank charted path for other openly gay lawmakers, 12/3/12

Barney Frank weds Jim Ready to become first congressman in same-sex marriage, 7/8/12

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