Republican Ted Cruz and Democratic challenger Bernie Sanders have snatched victories in the crucial Wisconsin presidential primary, sending a message to frontrunners Donald Trump and Hillary Clinton that their march to the nominations is not a foregone conclusion.

The United States networks -- including Fox News and NBC -- called the race for the two underdogs within half an hour of polls closing in the Midwestern state that borders Canada on Wednesday.

With seven per cent of precincts reporting, Cruz, a conservative senator from Texas, was ahead with 49 per cent of the vote compared to 35 per cent for billionaire Trump.

Cruz's win in the Badger State is a serious blow to Trump and could alter the trajectory of the Republican race.

The New York real estate mogul predicted a surprise win on Wednesday in Wisconsin, but now faces a narrower path to victory and the 1,237 delegates needed to sew up the Republican nomination.

On the Democratic side, Vermont Senator Sanders led with 57 per cent versus 43 per cent for Clinton. But the former secretary of state 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.