Joe Biden’s mad as hell and he’s not going to take, er … being the frontrunner any more. The former VP promised to come out swinging in the next debate, telling reporters yesterday that “I’m not going to be as polite” when attacked by his fellow Democrats. If his opponents want to dig into history, well, Biden’s totally up for that too, starting with Kamala Harris:

“I’m not going to be as polite this time,” Biden told supporters at a fundraiser in Detroit on Wednesday evening, according to a pool report. “Because this is the same person who asked me to come to California and nominate her in her convention.”

The former vice president hinted that he would cash in on his warning from earlier in July that he has “all this information about other people’s pasts.”

“If they want to argue about the past, I can do that,” Biden said at the fundraiser. “I got a past I’m proud of. They got a past that’s not quite so good.”

Harris isn’t the only one who’ll be seeing Biden’s evil campaign twin on stage. After Cory Booker attacked him for “mass incarceration” following the 1994 Crime Bill, Biden reminded audiences of Booker’s record as mayor in Newark. Policing got so harsh on minority communities, Biden noted, that the Obama administration had to take control of it:

On Wednesday, asked by NAACP panel moderator April Ryan if his views have evolved, Biden said the focus of criminal justice needed to shift from “incarceration to rehabilitation” — a solution that Booker, speaking to reporters after his earlier NAACP appearance, called “inadequate.” He described Biden as “an architect of mass incarceration.”

After Biden left the stage, he responded emphatically to Booker’s second day of criticism by trying to turn attention to his tenure as mayor of the troubled city of Newark, before he became a senator.

“Cory knows that’s not true, number one. Number two, you know the significant portion of the incarceration that occurred before the crime bill was written,” Biden told reporters before taking a shot at police practices during Booker’s tenure.

“His police department was stopping and frisking people, mostly African American men,” he said, leading the Obama administration to intercede. “We took action against them; the Justice Department took action against them, held the police department accountable.

“He objected to federal interference. If he wants to go back and talk about records, I’m happy to do that. But I’d rather talk about the future . . . I challenge him or anyone else to tell me how he has a better plan than I have for moving from here.”

So much for the rise-above-it strategy. But is it too early to abandon that as the smart way to protect his lead? After all, while RCP’s polling aggregation does show a decline in Biden’s polling, he’s still way out in front:

Biden’s nearly rebounded to his pre-announcement level of support even after the debate took some of the wind out of his sails. That still leaves him nearly 15 points ahead of a three-candidate knot consisting of Bernie Sanders, Elizabeth Warren, and Harris, whose fortunes have also begun sliding a bit since her debate bump. How much sense does it make to fight Harris on her terms, let alone Booker, whose current RCP aggregate is 1.6%?

Of course, Donald Trump has rewritten conventional political wisdom about punching down, but Biden’s no Donald Trump. His advantage in 2020 is posing as the Democrats’ eminence grise, the man who can unite everyone and return America to normalcy after its populist convulsions of the past four years. Getting into a fight over whether he or Booker is the biggest racist may be lots and lots of fun for Republicans, but it’s not going to do much for the kind of brand that Biden would need in a general election.