As a Sept. 30 budget deadline approaches, many in the Republican Party are pushing to repeal the Affordable Care Act, better known as Obamacare. The law's most radical opponents are even threatening to shut down the government by refusing to pass a continuing resolution, which would fund existing government programs at current or slightly adjusted levels. The political flaws of this strategy have been debated endlessly inside the GOP. But there is another important consideration: This doesn't have to be Republicans' "last stand." Obamacare doesn't have to be rolled back from without; it can be replaced from within. And Medicare, perhaps surprisingly, shows the way.