There will be plenty of time, likely seven months' worth, to predict the electoral consequences of each major party nominating a historically unpopular candidate for president. But as a starting point, it's worth noting the relative standings of Hillary Clinton (unliked), Donald Trump (unpalatable) and Ted Cruz (uncertain, but unloved so far).
In an Associated Press poll released this week, the three candidates most likely to appear on a general election ballot are all underwater with the public. That goes for their favorability:

As well as how many potential supporters they would attract:

These two poll questions present each of the candidates with results that would be devastating in a typical election, in which at least someone in the race wasn't roundly doubted or disliked. But you can insert the cliche for yourself. Clinton, Trump and Cruz have the benefit of having each other: each one with weaknesses, and each one potentially beatable.
Let's look at the first batch of numbers presented in this post. The overall takeaway is that Trump is in irrevocably bad graces with Americans, Cruz doesn't appear to be far behind, and Clinton has her own ditch to escape. Pit against even a mediocre opponent -- one with lukewarm support and without significant negative feedback -- all of these contenders would start from behind in a general election. (It's no surprise that John Kasich fares so well against Clinton in general election polling, and Bernie Sanders, he of socialism fame, dominates Trump and Cruz in one-on-one match-ups.)
But the intensity of the skepticism facing each candidate is a significant statistic here. 56 percent of respondents hold a "very unfavorable" opinion of Trump, which is ruinous, if not shocking given the developments of his campaign. Clinton also skates on thin ice, with 38 percent possessing a very unfavorable view of her. Comparatively speaking, the winner here is Cruz, of whom "only" 32 percent of those surveyed have a very unfavorable assessment.
Combining the percentages of survey-takers who say they have a "somewhat unfavorable" opinion of the candidate -- what we can call "soft" disapproval -- and those who "don't know enough to say", we have this:
Cruz: 41 percent
Clinton: 21 percent
Trump: 17 percent
Putting that into words, Cruz has the most room to grow with the public, at least according to these numbers. And it's not close.
The Washington Examiner's Philip Klein made a similar case based on different poll statistics and the fundamentals of the current race:
There has been a lot of talk about how Trump would go into the general election as the least popular nominee in decades of polling. But that amazing stat helped obscure the fact that other than Trump, Clinton would be the most unpopular dating back to 1984, in New York Times/CBS polling. Clinton remains mired in scandal and is still struggling to fend off a challenge from a septuagenarian socialist who she initially led by over 50 points nationally. As unpopular as Cruz may be with most registered voters, polling shows him within the margin of error nationally — and some polls taken in the last month have shown Cruz tied with Clinton in Pennsylvania, Ohio, Wisconsin and Florida. ... [T]he assumption is that Cruz cannot improve his image among the broader electorate, but that's hard to know for sure, because he's never had to do it. While opinions on Clinton are deeply entrenched after her decades in the public spotlight, Cruz isn't as universally known and has more of an opening to get a second look.
Moving on to the second set of numbers in this post, the openness of the public to supporting one of the three candidates, we get the impression that something's got to give. A majority of respondents claim they would "definitely" not back Trump or Cruz in a general election, which is a terrible result for each. But a plurality and near-majority say they wouldn't ever back Clinton, either, at 49 percent.
There's been plenty made about the polarization of the electorate. Here's yet another example. But this time around, the divisions run so deep that there's no telling who the dissatisfied voters might run to in November.
And there are a lot of them.