Walker on Tuesday called Cruz "a principled constitutional conservative who can win."
"After all these years of the Obama-Clinton failures, it’s time we elect a strong new leader, and I’ve chosen to endorse Ted Cruz," he said.
The Wisconsin governor, himself a former presidential candidate, announced his support on “Midday with Charlie Sykes,” a popular conservative radio show in the state. Sykes endorsed Cruz earlier this month.
Walker had once been seen as a top-tier presidential candidate, briefly topping the polls early last summer. But his lead, and his fundraising, quickly evaporated and caused him to drop out in September.
In that September exit, he explicitly called for the party to coalesce around a Trump alternative, warning that a “limited number of candidates” taking on Trump is “fundamentally important to the future of the party.”
Few heeded that call, leaving a divided field that never caught up to Trump.
Walker had been hinting at endorsing for the past few weeks, telling Sykes last week that he wanted to time his pick for “maximum impact.”
But Walker repeatedly refused to take the bait on Tuesday when Sykes asked him to contrast Cruz with Trump, framing the endorsement with a positive message instead of a critique of the GOP front-runner.
"I wanted to make sure I was supporting something, I wasn’t against someone or someone, but for something," Walker said of his endorsement.
"Americans want to know what you are for, not what you are against."
The governor is still a major player in Wisconsin politics and has a vast political network after surviving three elections in a span of four years — his initial election in 2010, a recall in 2012 and reelection in 2014.
Walker’s endorsement has the potential to shift the dynamic in the state, which has 42 delegates up for grabs. But Matt Mackowiak, a Republican strategist and contributor for The Hill, said the true effect will depend on how active Walker is in support of Cruz over the coming days.
“Walker’s endorsement would and should move dozens of state legislators — it’s the kind of thing that should really move numbers really quickly,” Mackowiak said ahead of the endorsement.
“If he endorses strongly, criticizes Trump, they get big media across the state and he brings his statewide organization with him, that would be hugely significant.”
Walker signaled to Sykes that he was "all-in" on the Cruz endorsement and will be campaigning with him at "a number of stops" in the run-up to the primary.
This report was updated at 10:45 a.m.