"I believe Ted Cruz is the candidate who is the answer to my prayers, a candidate God will use to restore the soul of America," King said.
King is a powerful voice in the first-in-the-nation caucus state popular with conservative Christians, and his support could be key to turning out Iowans for Cruz on caucus night February 1. Along with evangelical leader Bob Vander Plaats, King is the most sought-after endorser among candidates of the right, and he carries considerable appeal among hardline activists on immigration.
And given how early King's endorsement comes in the cycle -- he traditionally has waited until quite late in the caucus season to send word -- it could give Cruz real lift in organizing northwest Iowa, where King lives.
"With King's endorsement, he will finish in the top three in Iowa. And that's all he needs," said Steve Deace, a popular Iowa radio host and a top Cruz ally in the state.
The endorsement is not surprising to political insiders in Iowa: King's son Jeff has been working for one of the super PACs backing Cruz since the summer, and Bill Anderson, a state senator and a top King ally, has backed Cruz as well. But it comes at a time when Cruz is building momentum and organization that are crucial to a strong performance.
Citing the need for a candidate who can energize Christian conservatives and "take on the Washington cartels," King said he came to his decision following a trip to the Middle East last week.
Cruz said he was "beyond honored" to receive King's endorsement, calling King a "courageous conservative."
Campaigning Monday in South Carolina, Cruz told reporters that he even believed King would have appeal outside Iowa and in South Carolina, where few voters know him.
"There's no doubt Steve King's support will have enormous influence," Cruz said, talking up King's relationship with the GOP base. "Once conservatives unite, it's game over."
GOP activist and blogger at TheIowaRepublican.com blog Craig Robinson said the endorsement helps build Cruz's narrative in Iowa.
"It's just kind of a validation of Cruz's conservative credentials and definitely underscores the talking point the Cruz campaign wants to build," Robinson said, noting that most King supporters in Iowa are likely already committed to caucusing for Cruz.
"The thing Cruz struggles with the most is that he doesn't make personal connections with people, so King might help him do some of that on the trail," he added.