Texas Senator Ted Cruz scored a commanding victory over Donald Trump in Wisconsin's presidential primary Tuesday, putting the Republican frontrunner on notice that his march to the nomination is not a done deal.

The Democratic frontrunner Hillary Clinton also crashed to defeat against upstart Senator Bernie Sanders, who has now won six of the last seven Democratic contests and can bolster his claim to be a viable alternative standard bearer to the former secretary of state.

But the results of Wisconsin are almost certainly more damaging for Trump, the brash billionaire who stormed to the fore last year but has suffered crippling campaign trail setbacks in recent weeks with a series of controversial comments about abortion, NATO and other issues.

Cruz capturing most of Wisconsin's 42 Republican delegates now makes it far less likely that Trump will win the 1,237 delegates necessary to secure the nomination outright, raising the prospect of a contested convention in July when the delegates meet to choose the party nominee.

Turning point, says Cruz

"Tonight is a turning point. It is a rallying cry," Cruz told cheering supporters in Milwaukee after he received a hug from Wisconsin Governor Scott Walker, one of several former 2016 presidential candidates who have endorsed Cruz.

"It is a call from the hardworking men and women of Wisconsin to the people of America."

Cruz and the anti-Trump movement had eyed Wisconsin, birthplace of the Republican Party, as a crucial firewall against the celebrity billionaire's march to become the GOP flagbearer. The Texas senator's win in Wisconsin could well alter the trajectory of the Republican race.

Tuesday also cements Cruz's status as the leading anti-Trump candidate, with Ohio Governor John Kasich far back in the nomination battle.

"As a result of tonight... I am more and more convinced that our campaign is going to earn the 1,237 delegates needed to win the nomination," Cruz said.

That goal will be extremely difficult to reach; experts have projected Cruz would need to win more than 80 percent of remaining delegates to reach the magic number.

US networks called the race for Cruz and Sanders within half an hour of polls closing in the Midwestern state that borders Canada.

With half percent of precincts reporting, Cruz, a conservative senator from Texas, was ahead with 51 percent of the vote compared to 32 percent for billionaire Trump. Kasich was a distant third at 14 percent.

On the Democratic side, Vermont Senator Sanders led with 55 to 44 percent over Clinton.

"This campaign is giving energy and enthusiasm to millions of Americans," Sanders told supporters in Wyoming, which holds a Democratic vote Saturday.

Sanders said his momentum gives him an "excellent chance" to win California, Oregon and other states.

"We have a path toward victory, a path toward the White House," he declared.

But Clinton, the onetime first lady who is aiming to become the nation's first-ever female commander in chief, can look forward with some confidence to the upcoming races.

She leads Sanders by double digits in New York, her adopted home state which votes April 19, and Pennsylvania, which casts ballots a week later.

Trump also leads handily in his home state of New York and in Pennsylvania.

Trump's sour grapes

His campaign for months had appeared immune to criticism or self-implosion, yet the brash billionaire went into Tuesday's vote having suffered a brutal week on the campaign trail, including comments expected to hurt him with women voters.

After the Wisconsin vote his team lashed out at Cruz, saying the senator had "the entire party apparatus behind him."

"Ted Cruz is a puppet -- he is a Trojan horse being used by the party bosses attempting to steal the nomination from Trump," said the campaign in a statement reported by MSNBC.

Trump had 740 delegates heading into Tuesday. Cruz had 474 and Kasich 145, according to CNN.

If none reaches the magic number of 1,237 before the Republican primary races wrap up on June 7, the nominee will be decided at a contested convention where, after the first ballot, delegates will be free to vote according to personal preference instead of being bound by the primary results.

Clinton has secured 1,742 delegates prior to Tuesday -- including 483 so-called "super-delegates" who are not bound by primary results -- while Sanders has 1,051 total, according to a CNN tally.

A candidate needs 2,383 delegates to secure the Democratic nomination.