A new TV ad from the conservative Club for Growth is urging Wisconsin Republicans to back Ted CruzRafael (Ted) Edward CruzWith religious liberty memo, Trump made America free to be faithful again Interstate compacts aren't the right way to fix occupational licensing laws Texas Dem: ‘I don’t know what to believe’ about what Trump wants for wall MORE and warning that a vote for John Kasich will only help GOP front-runner Donald TrumpDonald John TrumpDems flip Wisconsin state Senate seat Sessions: 'We should be like Canada' in how we take in immigrants GOP rep: 'Sheet metal and garbage' everywhere in Haiti MORE.

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“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.