Yes, Cruz undermined Trump by not endorsing him, and I think that's a good thing for the Republican Party. It shows that there are still some sensible Republicans who can think for themselves. While he may have undermined Trump, he certainly boosted his party, and the Republican Party definitely needs a boost right now.
Not only did Ted Cruz undermine Donald Trump but he also verbally gave him the finger. He came to the convention which was supposed to be there to honor Donald Trump and essentially told everyone to "vote their conscience." This was obviously prompting them to vote for someone else. He stood firm when the crowd demanded him to endorse Trump. It was personal for him. He obviously wanted Donald to know how much he detested him.
Ted Cruz made headlines when he refused to endorse Donald Trump for president. While some might consider this a serious blow to Trump's chances, I disagree. Trump has become such an incredible figure in his own right that he has little need for the opinions or endorsements of other politicians.
I think that Cruz was sleazy for not endorsing Donald Trump, but he didn't undermine Trump's campaign by his odd speech. I think that most voters see Cruz for exactly who he is, and that is why he didn't even come close to challenging Trump in the primaries. If anything, Cruz just hurt himself.