From Nixon onwards, Republicans have won election after election by wrapping themselves in the flag, taking a hard line on immigration, and running ads with scary black men in them. In doing so, of course, they've relied very heavily on white votes to see them to victory. Back in the 1970s only about 10% of voters were non-white, so the GOP could afford to write them off. These days, that figure is more like 25%. And, unsurprisingly, non-whites are still more inclined to vote Democrat. If you're a Republican, this is a problem. Here's Nate Silver:
In 1980, 32 percent of the electorate consisted of white Democrats (or at least white Carter voters) -- likewise, in 2008, 32 percent of the electorate consisted of white Obama voters. But whereas, in 1980, just 9 percent of the electorate were nonwhite Carter voters, 21 percent of the electorate were nonwhite Obama voters last year. Thus, Carter went down to a landslide defeat, whereas Obama defeated John McCain by a healthy margin.
In other words, the dice are now loaded in favour of the Democrats. Replay the 1988 election with today's demographics, and even Michael Dukakis coulda been a contender.