A new TV ad from the conservative Club for Growth is urging Wisconsin Republicans to back Ted CruzTed CruzKoch-backed group launches new ads on tax reform Healthcare wish lists: What moderates, conservatives want Overnight Healthcare: GOP infighting erupts over bill | How Republican governors could bring down ObamaCare repeal | Schumer asks Trump to meet with Dems MORE and warning that a vote for John Kasich will only help GOP front-runner Donald TrumpDonald TrumpDemocrats target Trump's border wall in defense bill debate Obama ethics czar: Trump fundraiser at his DC hotel ‘illegal’ Trump trolling of Comey — Not presidential MORE.

“If you don’t want Donald Trump to win, your choice comes down to this: math,” the ad says. "Only Ted Cruz can beat Trump. John Kasich can’t do it. The math won’t work.

“A vote for Kasich actually helps Trump by dividing the opposition. It’s time to put differences aside. To stop Trump, vote for Cruz.”

The new spot, released Thursday, begins airing Saturday in Wisconsin ahead of the state's April 5 primary.

Club for Growth says it's spending $1 million to air the 30-second spot on digital and TV outlets.

“Donald Trump is the worst candidate in the Republican field, and he can’t win in November,” said David McIntosh, Club for Growth’s president in a statement. "It’s time for Republicans to rally around Ted Cruz, the one candidate who can lead the party to victory.”

Club for Growth’s political arm on late Tuesday formally endorsed Cruz, the group’s first presidential primary endorsement ever.

“Their records make clear that Ted Cruz is a consistent conservative who will fight to shrink the federal footprint, while Donald Trump would seek to remake government in his desired image,” McIntosh said.

Club for Growth has put Trump in its crosshairs, releasing ads attacking his healthcare plan and past remarks concerning women.

New Day for America, a pro-Kasich group, hit back at the ad. Matt David, the group's chief strategist, dismissed Club for Growth as a "special interest group run by a bunch of Washington, DC hacks and has-beens," in a statement.

"And, anyone being honest with themselves knows Ted Cruz will not get to 1,237, but honesty has never been their priority — their agenda is their priority," he added.

Trump leads the delegate race for the Republican presidential nomination, boasting 739 delegates to Cruz’s 465 and Kasich’s 143. A candidate can clinch the nomination before the GOP convention in July with 1,237 delegates.

Wisconsin holds its GOP presidential primary on April 5. Badger State voters will award 42 Republican delegates in a winner-take-all contest.

—This story was updated at 5:29 p.m.