As partisan as anyone may be, this is a very good question right now, mostly because Donald Trump, an individual who would not be electable in any other time in history, has a very good shot at a win. If Bernie Sanders gets the nomination, he will beat Trump easily in the general election. There is no real dirt on Sanders, which is going to make smear ads ineffective for the most part. Trump wouldn't be able to launch an effective negative ad campaign, and Sanders would also pull a signifcant amount of the discontent vote from Trump, as the more likeable of the two candidates.

However, if Hillary Clinton gets the nomination, the election is going to be much closer, at least in terms of the popular vote (the electoral vote will probably have a significant spread). Trump appeals to a large population of the rust belt which contains several huge swing states. He demonstrated his ability to capture votes in probably his toughest terrain in Indiana when he resoundly beat Ted Cruz and his fundamentalist platform.

Trump's biggest and most insurmountable problem is with his approval rating among women, who disfavor him by about 70%. So while he has swing state appeal from a policy perspective, he loses a decisive swing vote with women. Clinton does not gain with women just because she is a woman, although it does help. Trump generally repulses women as a result of years of derogatory comments about various public women.

In the end, a Democrat will be President this year, and because she is the Democrat establishment pick, it will most likely be Hillary Clinton.