Not too long ago, the polls indicated that if the general election came down to a choice between Donald Trump and Hillary Clinton, Clinton would win in a landslide. Then Donald Trump secured the Republican nomination, large numbers of Republicans who had opposed him offered their (often grudging) support, and the polls show that he has eliminated the gap. Some polls, in fact, are now showing Trump winning the not-entirely-hypothetical matchup—and the trend is certainly in his favor.
Democrats are likely counting on Clinton getting a similar "party unity bump" when Bernie Sanders finally bows out of their race, viewing Trump's sudden competitiveness as an artifact of his having unified his party first. They are likely to be disappointed.
Contrary to conventional wisdom, Trump faces the easier unification task. First, as in most elections, the party that has been out of power for eight years is the hungrier of the two. President Obama has already given the Democrats many of the things they consider most important; their passion for transgender bathrooms simply cannot match the importance they placed on centralizing health care.