Indiana Gov. Mike Pence made his support for the Texas senator official during an afternoon radio interview, casting Cruz as "a principled conservative."
"The man has shown the courage of his convictions," Pence said, citing Cruz's fight against government spending and the federal health care law, and his "strong and unwavering stand for the sanctity of life."
Pence, who faces his own re-election test this fall, also praised Trump, who he said "has given voice to the frustration of millions of working Americans." ''I'm grateful for his voice in the national debate," he said.
"I'm not against anybody, but I will be voting for Ted Cruz in the upcoming Republican primary," Pence added.
The Indiana governor's backing could give Cruz a desperately needed boost in his fight to block Trump from claiming the delegate majority before the GOP's national convention in July. A Trump win in Indiana on Tuesday would all but ensure he becomes the presumptive nominee.
Trump swept all five Northeastern primary elections earlier in the week and enjoys a massive delegate advantage over his Republican rivals. Cruz has been mathematically eliminated from earning the 1,237 delegate majority, but insists he can block Trump from the majority as well, as the 2016 contest shifts to "friendlier terrain" in the West and Midwest. The Texas senator declared he is "all in" on Indiana.
After his latest round of losses, Cruz bid to regain some footing by making an early announcement of his running mate, tapping Carly Fiorina, the former Hewlett-Packard executive who dropped out of the GOP race earlier this year.
His campaign says it raised $2 million around that announcement. Campaign manager Jeff Roe confirmed the fundraising total to The Associated Press.
Earlier Friday, Cruz said he would "enthusiastically welcome" Pence's support.
"The country is depending on Indiana to bring some sober common sense," Cruz told reporters in Indianapolis, "instead of going down a rash course of action that is endangering this country."
Pence had been under enormous pressure from pro- and anti-Trump factions. Although he is more closely aligned with Cruz, he risks voter backlash in the fall if Tuesday's primary contest shows Indiana is filled with Trump voters.
Trump said this week that he had met the governor and asked for his backing. He said he didn't know if he would get it but did not expect Pence to come out in support of Cruz.
Yet Cruz and conservatives who support him pressed Pence from the other direction.
"Every day he sits on the sidelines is another day in which he could have made a difference," Republican columnist Erick Erickson wrote Thursday on the website The Resurgent. "He has not used his influence in the conservative movement to rally against Trump."