Matt Steinglass says that all the talk about Barack Obama being a socialist or Mitt Romney being a social Darwinist is a "reverse dog whistle." These aren't words with subtle meanings that your own supporters understand but no one else does, they're words designed simply to piss off your opponents. And it works! When you fight back against this stuff, you lose:

What liberals say: Barack Obama is not a socialist! Socialism is government control over the entire economy, not bailouts of private banks and industries that leave them private, like Obama's (which Bush started anyway)! Obamacare isn't a government takeover of health-care, it's based entirely on private insurers! That's less socialist than Medicare!
What voters hear: Obama...socialist....socialism...bailouts...Obama...Obamacare...government takeover...socialist.

What conservatives say: Mitt Romney is not a social Darwinist! He's a middle-of-the-road Wall Street executive! Just because his business success has made him rich doesn't mean he doesn't care about poor people! Social Darwinists believe poor people are inherently inferior to rich people; Romney doesn't believe that, he thinks deregulation and tax cuts will empower the poor to better themselves! Recognising that we need to cut Medicare spending growth doesn't make you a social Darwinist, Romney's just recognising budgetary reality!
What voters hear: Romney...social Darwinist...Wall Street...rich...social Darwinist...poor people are inferior...cut Medicare...Romney.

As the old saying goes, If you're explaining, you're losing. Or, more pungently, there's the (possibly true!) story about LBJ spreading a rumor that his opponent was a pig-fucker. Aide: "Lyndon, you know he doesn't do that!" Johnson: "I know. I just want to make him deny it." If you're denying, you're losing.