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For the last several years, the United States has essentially had three political parties: progressive Democrats, moderate Democrats and extremely conservative Republicans. Even the Republicans who aren't extremely conservative have to behave like extremists to avoid inflaming their base. By labeling them "extreme conservatives," Democrats put them in the uncomfortable position of either accepting the "extreme" label or replying that they are not extremely conservative, thereby incurring the wrath of the tea party.
Additionally, with this three-party framing, Democrats in conservative districts can actually run against the president in places where he's unpopular: "I'm more of a Reagan Democrat than an Obama Democrat; the difference between my opponent and me is that I want to reach across the divide and find solutions instead of trying to blow up the government."
This loyal opposition approach can work on all kinds of specific issues, including the Affordable Care Act: "The left wants to keep Obamacare the way it is; the right wants to get rid of it altogether. I say let's keep what's working and fix what's not."
Above: Tea party firebrand Sen. Ted Cruz of Texas.