Cruz is largely playing that role in the lead-up to the April 5 primary in Wisconsin, where the two most recent polls showed him with single-digit leads over Trump.
The Republican front-runner tried to lay some groundwork for his campaign in interviews Monday with three of the state's leading conservative talkers, including WTMJ radio's Charlie Sykes - to whom Trump confessed on-air not knowing that Sykes is a leading voice against his candidacy.
CNN is hosting a town hall event on Tuesday evening ahead of a crucial primary in Wisconsin next week.
Last summer, Trump attacked Walker, saying "Wisconsin is doing terribly" under him, citing problems with the state budget.
"I'm all in", Walker said.
Trump called his actions a "minor response" to the group's ad and said he has "been better to women than any of these candidates, frankly".
On the Democratic side, former Secretary of State Hillary Clinton will visit Milwaukee, La Crosse and Green Bay. But rival Ted Cruz has gotten a jumpstart on the contest, racking up influential endorsements, campaigning in key regions and supported by bullish advertising campaign.
Meanwhile, Cruz heaped praise on his former rival, calling Walker's fight against labour unions "inspired people all over the country". Graham, who dropped his own bid for the White House a year ago, told The Associated Press Tuesday that Trump does not understand the stakes of the Middle East. He took particular aim at Trump's assertion that he would take a more neutral stand regarding the Israeli-Palestinian conflict. "We are running for president", said Mr Sykes. Ohio Governor John Kasich brings up the rear at 20.8 percent.
The case of Lindsay Graham, who is known to have said, "If you killed Ted Cruz on the floor of the Senate, and the trial was in the Senate, nobody would convict you", shows the level of desperation among the GOP.
The importance of winning Wisconsin has grown significantly as Trump tries to reach the necessary 1,237 delegates needed to win the nomination without a contested convention.
Sykes had asked Trump earlier in the interview if he would apologize for indirectly mocking Cruz's wife. The latter means that it was time at last for Scott Walker, the goggle-eyed homunculus hired by Koch Industries to manage this particular subsidiary, to muster his mighty 39 percent approval rating behind his candidate of choice.