Despite pledging to embrace whomever wins the Republican presidential nomination early on in the election, Donald Trump retracted that promise Tuesday. When asked if he still stood by the loyalty pledge he signed in September at a CNN town hall meeting, he responded, "No, I don't anymore." The real estate magnate going back on his word isn't exactly surprising to anyone, but Trump's refusal to support any nominee other than himself is excellent news for the Democratic Party.
As it becomes more and more likely that there won't be a clear winner at the first round of voting at the Republican National Convention in July, efforts to ensure a contested convention are ramping up. For instance, former candidate Marco Rubio has asked for the delegates he won to remain bound to him. Cruz is confident that he'll beat Trump, answering the same town hall question about standing by the loyalty pledge by saying: "Let me tell you my solution to that — Donald is not going to be the GOP nominee. We're going to beat him."
Trump claimed that he doesn't need Cruz's support, saying he doesn't want Cruz "to be uncomfortable" backing him. It's very possible that he would say the same thing of the GOP leadership if he loses the nomination, running as an independent counter to the Republican and Democratic nominees.
This same scenario was a fear early on, when Trump initially refused to sign the loyalty pledge. But him finally accepting the terms gave him more validity as a Republican candidate. Now that his following is larger than expected, an independent Trump candidacy is even more dangerous for the party. Trump running against Cruz in the general election would disastrously split the party's voters, making it much easier for Hillary Clinton or Bernie Sanders to win the presidency. The logic is simple: Two quasi-Republican candidates running against one another would force right-wing voters to choose between the two, and no matter which one got more of those votes, it wouldn't be enough to crush a Democratic candidate with an entire party behind them.
Trump's supporters revel in the fact that he's the anti-establishment, non-politician candidate, so they aren't likely to switch their support to Cruz to keep the party together. They don't like the party — they like Trump, which is precisely why him mounting an independent campaign would be so detrimental. This should make Democrats very happy. The party wouldn't even have to do much to win, considering how the GOP would be self-destructing from within. An opponent competing with itself makes for an easy match.