Joe Biden took to an Iowa podium on Wednesday ­afternoon and made it clear that he is going to continue treating his presidential bid as a two-candidate race — Joe Biden vs. President Trump.

As a matter of simple politics, it’s not only the best path for the former veep — it is the only path.

Using his signature theme — “we are in a battle for the soul of this country” — Biden went after the president for “pouring fuel on the fire” of white supremacy and contrasted the Obama-Biden administration’s behavior with Trump’s.

“Our president isn’t up to the moment,” he said. “Our president has more in common with George Wallace” — the staunch segregationist and failed presidential aspirant — “than George Washington.”

You might think that unfair. You might think it ahistorical. But I’m afraid it’s hard to complain about rhetorical mistreatment when the subject of Biden’s attack is the man who suggested that Sen. Ted Cruz’s ­father may have been involved in the Kennedy assassination and who said of another GOP rival, Carly Fiorina, “look at that face!”

No, when considering what Biden has done and is doing, the real question is whether the case he makes against Trump is effective enough to convince fence-sitting Democrats to move toward him in the primaries — and whether what he says resonates with the Democratic voters in Pennsylvania, Michigan and Wisconsin who stayed home in 2016 and thereby handed Trump the presidency.

If you consider how focused Biden is on selling himself as the best and most august alternative to the president, you can begin to understand his plodding behavior in the two Democratic ­debates in which he ­recently participated.

He doesn’t want to fight with his Democratic rivals. He wants to draw contrasts with his rivals where necessary but sees no profit in turning on them the way they are turning on each other. He wants to be the person who unites the party without having to fight trench warfare to get there.

And guess what? It’s working.

There is a peculiar idea out there that Joe Biden is a weak frontrunner. Oh? He has been in the Dem presidential primary race formally since the end of April and informally since the beginning of the year. That is seven months. And he has led. In the polls. Every ­single day.

People say it’s early. People say there is a lot of time for one or two contenders to rise and challenge him. Well, sure. But it isn’t really early, and there’s not a lot of time.

Remember how, in 2016, commentators like me imagined it was early when Donald Trump rose to the top in the third week of July 2015, and that one of the more mainstream candidates in the Republican primary would surely rise and lap the New York developer-turned-TV star? Didn’t happen.

Trump had only been in the race a month, but the other GOP candidates had been running for six months — and enough Republican voters had seen them and decided they were “meh” and that Trump was their guy.

Aside from one errant poll that showed Ben Carson tied with him, Trump never surrendered his poll lead from July 20, 2015, onward.

Democrats know Sen. Elizabeth Warren. They know Sen. Bernie Sanders. The two hard-left firebrands are running second and third. And since they are running with remarkably similar agendas and populist messages, it would stand to reason that if one were to drop out, the other would simply collect all those votes.

And if that were to happen, which it won’t, the total would . . . just about match Biden’s support in the polls.

We saw this too with the Republicans in 2015 and 2016. Pundits like me kept saying, “Look, Trump only has 25 percent — no, wait, make that 30 percent — no, wait, make that 35 percent of the Republican vote. The other 75, 70, 65 percent don’t want him.”

Well, it turned out that 35 percent support is more than enough (and climbs once the voting starts) in a field with 97 million other candidates.

All in all, Joe Biden’s speech on Wednesday was certainly the best he has ever delivered, and will only enhance his standing as the man to beat. I sure hope it wasn’t plagiarized from Neil Kinnock.

John Podhoretz is editor of Commentary. jpodhoretz@gmail.com