Who knows if I am right, but I think this is good news for the Democrats' prospects in the November election. Hillary Clinton (who I am confident will be the Democrat's nominee) should easily beat Trump or Cruz in the general election. The electoral map simply favors any Democrat in a presidential race. Both Trump and Cruz happen to have even higher negative approval ratings than Clinton, so it should come out in favor of the democratic victory no matter which one runs.
But there is a difference. If Cruz is the nominee, I think there is a not-insignificant chance that Trump would run as an independent or third party candidate. While that would make it even more unlikely for the Republicans to take the presidency (and Trump and Cruz would likely split the red vote), it might hurt the democrats overall compared with the scenario where Trump is the nominee.
If Trump is the nominee, his outrageous and unpopular behavior is going to dog all the Republicans running this year. When Trump inevitably makes some over-the-top sexist remark about Hillary Clinton, all the other Republican candidates are going to be asked whether they support those remarks, which will force them to either criticize their party's nominee in a hard fought presidential race, or endorse something that will hurt them personally. Trump also will be a great motivator for Democrats to turn out in the general election. That increased motivation and turnout is not just likely to help Democrats win the White House, it probably will put them in control of the Senate and to make some gains in the House (although I doubt if it will be enough to put the House back into Democrat's control, though that would be a possibility) It might also pay dividends in various State races, tipping things back a bit from the Republicans' utter domination of State governments.
If Cruz is the nominee and Trump runs as a third party candidate or independent, it will be a lot easier for Republicans to disavow whatever dumb shit that Trump says. While Cruz and Trump will lose the presidency, having two conservative candidates in the general election is going to boost Republican turnout. I think that will blunt democratic gains in the down ballot races.