Some Supreme Court rulings end conversations, and some start them. The court's decision on gay marriage is an ending: It caps an era in which public support for marriage equality has undergone a meteoric rise, a development that will likely protect the decision from political backlash. Ask national Republican politicians off the record, and most will tell you they're relieved to have gay marriage off the table.

But King v. Burwell, the ruling that saved Obamacare, is more of a beginning. If Burwell brings to a close an era in which Obamacare looked reversible, it opens a much more interesting argument, one in which the two parties must compete to shape a national healthcare system that may be the law of the land, but remains deeply flawed and highly malleable. And the key to winning this argument is remembering that Americans have exceptionally short political memories.

In a handful of years, maybe as soon as Election Day 2016, it's going to be politically irrelevant that Democrats passed Obamacare on nearly party-line votes, or that Republicans spent half a decade working to dismantle it. All that's going to matter is who can best answer the question: Now what?

Democrats neglect this at their peril. If they decide to take Burwell as permission to rest on their laurels, they're forgetting the lesson of Ronald Reagan, who made himself a plausible candidate by coming to terms with the legacy of the New Deal, and of Bill Clinton, who rose from political oblivion by co-opting the conservative position on welfare reform.

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