We’re not sure what President Barack Obama’s legacy will be. But we are sure about what it should be:

Presidents should not pass major, once-in-a-generation legislation without bipartisan support.

When Massachusetts voters elected Scott Brown to the U.S. Senate in 2010, Obama faced a choice. A few weeks earlier, Senate Democrats had passed their version of Obamacare. They’d taken advantage of their 60-seat, filibuster-proof majority to pass the massive bill on a party-line vote.

But now, here was Massachusetts, rebuking Democrats by electing Brown – a Republican – to the late Sen. Ted Kennedy’s seat.

And with that, Senate Democrats lost their 60-vote supermajority as well as any pretense to a national consensus in favor of health care reform.

How to respond?

That was the choice Obama faced – and he chose poorly. He ignored Massachusetts’ warning and pushed ahead with the reform. And as a result, the Affordable Care Act became law without a single Republican voting in support.

He still hasn’t lived that mistake down. Because the partisan passage turned Republicans into enemies of the act, not grudging skeptics who’d eventually lend fuller support.

That meant the act’s days were numbered from the start. Because sooner or later in America’s two party system, each party gets to have its way. Republicans’ way is coming on Jan. 20 – and one of their first orders of business is Obamacare’s repeal.

That’s new in American politics, in large part because previous leaders were more sensitive to this dynamic. For example, President Lyndon Johnson – despite enjoying overwhelming Democratic majorities in both houses of Congress – refused to settle for partisan passage of his Great Society reforms.

Johnson courted Republicans because “he saw bipartisan support as an essential foundation on which to build lasting commitment among the American people,” recalled Joe Califano Jr., Johnson’s secretary of health, education and welfare, in 2008.

“He knew that the endurance of his legislative achievements, and their enthusiastic acceptance by state and local governments, powerful private interests and individual citizens across the nation, required such bipartisan support.”

Or as presidential historian Arthur Schlesinger Jr. once put it, “L.B.J. has no idea of his own but consensus.”

Obamacare passed on an all-Democratic vote, and we won’t be surprised if it’s killed through an all-Republican effort. But Donald Trump should learn his own lesson from Obamacare’s death, and insist on bipartisan support for his own signature initiatives. That’s the way America passes legislation for the ages, not just a few presidential terms. That’s the way to win over a supermajority of the public, which is the kind of support that landmark legislation needs.