Passage of President Obama’s signature achievement -- The Affordable Care Act (ACA) -- took a heavy toll on his presidency, cementing a partisan divide that dogged his two terms. Obama may get some measure of revenge as the Republicans and President-elect Trump wade into the briar patch of repeal-and-replace with no agreed-upon alternative. It’s theoretically easier to revise something that already exists, but Republicans risk alienating millions of Americans who depend on the ACA if they can’t come up with something clearly better. This debate will dominate the early months of Trump’s presidency. The result will define the Republican Party for decades.