As an American, I wish President Joe Biden every success in defeating the pandemic and getting the economy moving again. But when it comes to the dysfunction of our political system and the polarized climate generally, it’s becoming clear he’s part of the problem, not the solution.

I don’t think this is deliberate. No doubt Mr. Biden believes he’s doing right as he sees it. He’s also probably sincere about his desire to detoxify our politics by pushing for some gauzy notion of “unity.”

The real problem isn’t the player, it’s the game. Over the last half-century, the parties (and their voters) have separated from each other like oil and water in a centrifuge.

As Amy Walter of the Cook Political Report notes, Americans “don’t even agree on the same set of problems.” She points to data from the Pew Research Center that illuminates the divide.

According to Pew, “In 1999, improving the educational system topped the list of priorities for both Republicans and Democrats, and four of the top five issues for Republicans were listed among the Democrats’ top five issues as well.”

“Ten years later,” Pew researchers report, “in the wake of the financial crisis, three issues (economy, jobs and terrorism) topped the list among both Republicans and Democrats (although they ranked them slightly differently).” As of 2019, there was “not one overlapping priority between the top five priorities for Democrats and those of Republicans.”

Heap as much scorn as you like on Donald Trump (I won’t object), but his legislative agenda was largely dedicated to giving Republicans what they wanted. It looked deeply partisan because it was. In a country where there is a lot of consensus about what to do, it’s easy for a president to pick an issue that’s popular in both parties. It’s harder when few such issues exist.

That’s one reason George W. Bush started his presidency by working with Sen. Ted Kennedy and the Democrats on education reform.

Mr. Biden isn’t following Bush’s example. He’s following his two immediate predecessors, Barack Obama and Mr. Trump. Though, so far, it’s mostly been through executive orders.

To be clear, I disagree with most of the policies Mr. Biden has announced, but my point isn’t about policy; it’s about politics. As it stands, it looks like the Biden administration would rather push a COVID-19 relief package on a straight party-line vote than agree to some bipartisan compromise package that didn’t “go big” as defined by the Democratic base.

Even if Mr. Biden is right about the need for a $1.9 trillion bill — I don’t think he is — the cost will be more than economic if he crams it through with only Democratic votes.

In 2009, Mr. Obama pushed through a stimulus bill with no Republican support in the House and only three Republicans in the Senate. At the time, Mr. Obama didn’t think he needed Republican support. And as a matter of vote-counting, he was right, given the Democratic majorities.

But he could have gotten Republican votes. Mr. Obama was very popular when he took office. According to Gallup, he started with an initial approval rating near 70%, and an astonishing 83% of Americans approved his transition efforts. Republican strategists at the time told me that if Mr. Obama had offered Republicans real buy-in, he easily could have gotten enough votes to make the stimulus bipartisan. Mr. Obama thought he already made concessions to the GOP on the substance. What he didn’t appreciate is that to get participation from the opposition, the opposition needs to be included in the process, not handed an ultimatum.

As a result, Mr. Obama owned the stimulus politically. And when the stimulus failed to stimulate, the Republicans could run against “the Obama economy” in the midterms. They did, leading to a landslide rout of the Democrats. The stimulus led directly to the GOP’s obstructionist stance throughout the Obama presidency.

When you push major legislation on a party-line basis, the opposing party has no incentive to provide cover for it. Moreover, as Ms. Walter writes, “it alienates not just those in the other party, but it may fail to attract independent voters. If only one side is willing to defend it — and the other side is busy trashing it — well, that law can become pretty unpopular, very quickly.”

That’s what happened again and again to both Mr. Obama and Mr. Trump.

Mr. Biden may not need Republicans to get the legislation he wants right now, but he needs at least some of them to get the “unifying” politics he wants.

Jonah Goldberg is editor-in-chief of The Dispatch and the host of The Remnant podcast. His Twitter handle is @JonahDispatch.