Key said that in Texas, the Republican candidate typically holds the advantage.
"The incumbent Republican historically is in the driver's seat," he said. "I would say honestly, it's not to a politician's best interest to run as a Democrat down here. But, you know, unfortunately I felt like that is what I had to do. This is a personal decision."
Key announced in a campaign ad on Monday that he was leaving the GOP and running for re-election as a Democrat due to "pettiness and bigotry" in the Republican party.
Key elaborated on CNN Tuesday about how Ted Cruz's push to defund Obamacare influenced his party switch. He added that Cruz's strategy "absolutely" factored into the decision.
"The tea party represents one of the most extreme positions of the Republican party. You know, down here in South Texas where I am, there's a lot of support for that," Key said. "It seems to be that Texas happens to be one of the ground zeros for the tea party. And it really is, you know, pushing the conversation to the extreme."