In an exclusive interview with Eyewitness News, Cruz said it was not a snap decision.
"There were no deals," he said. "We had been engaged in conversations. You know it's been a decision as I said I've been thinking about and praying about for weeks and months, discussing it with my family, and I made the decision today and announced it Friday.
The first-term Texas senator took a lot of heat when he did not endorse nominee Donald Trump at the Republican National Convention this summer.
Even supporters were angry he wasn't backing Trump despite promising to do back whoever the eventual nominee was.
Now he's backing a man who he's called "pathological liar", "a narcissist" and a "sniveling coward."
"Listen," he said. "(In) politics, there's always criticism. If no one is throwing rocks at you, you're not doing much of anything. I've discussed it with both Heidi (Cruz) and my dad. I love my wife. I love my dad. Both of them have forgiven Donald. I have forgiven him. This is not personal. This is not about our family. It's about the country. "
For his part, Trump said he wouldn't accept Cruz's nomination if offered, but today, he too changed course-releasing a statement that reads, "I am greatly honored by the endorsement of Senator Cruz. We have fought the battle and he was a tough and brilliant opponent. I look forward to working with him for many years to come in order to make America great again.
Cruz said he has offered his help to the Trump campaign, but there are no formal plans for any campaigning yet.
Source says it was Trump's support of Internet freedom push and Supreme Court list that pushed endorsement. https://t.co/zzJWW8bCy0— Tom Abrahams (@tomabrahams) September 23, 2016
Source familiar with decision says Cruz did as promised. "He watched and listened to candidates" before deciding https://t.co/N9pSavLXx9— Tom Abrahams (@tomabrahams) September 23, 2016
"We have to choices," he said. "We have Hillary or Donald. Now when this race started I had another choice in mind. I campaigned very hard. Tried to make that case. Out of 17 republican candidates, we beat 15 of them, but we didn't beat 16 of them and at this point Donald is the republican nominee. At this point it is a choice, a binary choice between Donald Trump and Hillary Clinton and my conscience leads me to conclude that a Hillary Clinton presidency would do enormous damage to the court, to the bill of rights, to the constitution, to our freedom, to our jobs. And that's why I am going to be voting for the republican nominee."