Why are the Dems and unions already going after Romney?

This should be a huge story. Obama and the Democrat National Committee and unions are spending lots of money attacking Mitt Romney in Florida. When Nixon's team did that in 1972, to promote the candidacy of George McGovern, whom Nixon beat handily, it was called "Dirty Tricks." Does this mean that the Dems think Romney would be their strongest opponent? Reporter Scott Powers of the Orlando Sentinel (not the New York Times, not the Washington Post, not the Wall Street Journal, etc.) broke the story here: http://www.orlandosentinel.com/news/politics/os-democrats-against-romney-20120124,0,7825399.story