Bernie Sanders and Ted Cruz won their Wisconsin presidential primary elections, prolonging the Democratic race and increasing the chances that the Republican nominee will be picked at a contested party convention in July.
The midwestern state, the latest qualifying round in the most unconventional race for the White House in generations, dealt losses to frontrunners, Republican Donald Trump and Democrat Hillary Clinton, as their main challengers scored big-margin victories.
With 92 per cent of votes counted, Mr Cruz was leading Mr Trump by 49 per cent to 35 per cent in Wisconsin's Republican primary with the only other candidate, Ohio governor John Kasich, winning just 14 per cent of the vote.
In the Democratic race, Mr Sanders held a lead of 56 per cent to 43 per cent over Mrs Clinton with the same percentage of votes counted.
Mr Cruz, the Republican senator from Texas, enjoyed the support of a coalition of conservative critics of Mr Trump and Republican establishment opponents seeking to halt the New York billionaire's march towards the required 1,237 party delegates he needs to clinch the nomination.
"Tonight is a turning point. It is a rallying cry," the 45-year-old senator told a victory rally in Milwaukee, pitching his victory and landslide win in Utah two weeks ago as turning points for the "Never Trump" movement.
The victory confirmed that the Texan, a conservative firebrand and unpopular figure among the Republican establishment, is the unlikely, chosen alternative for party’s senior figures to the insurgent Trump.
The Wisconsin outcome drew an uncharacteristically muted response from Mr Trump who chose to release a statement attacking his chief rival rather than appearing before television cameras after a heavy defeat.
The businessman angrily denounced his Republican opponent calling him “Lyin’ Ted Cruz” and attributing his victory to the support of conservative talk radio show hosts and “the entire party apparatus behind him”.
“Ted Cruz is worse than a puppet - he is a Trojan horse, being used by the party bosses attempting to steal the nomination from Mr Trump,” said the statement released by the businessman’s campaign team.
Exit polls showed that Mr Cruz won amongst men and women, almost all age groups as well as among college graduates and blue-collar workers, a key voter group that has handed Mr Trump a strong lead among delegates.
The Texan scored strongly with voters who were dissatisfied with the government, breaking Mr Trump’s winning streak among angry voters.
The Wisconsin upset for the property developer-turned-reality TV star leaves him facing an uphill battle in the remaining 16 state contests.
It was estimated that Mr Cruz collect at least 33 of the 42 delegates from his victory in Wisconsin, not enough to close Mr Trump’s lead of more than 200 delegates but enough to raise the likelihood that he would block the businessman winning a majority in state contests before the convention.
Mr Kasich is on course to pick up no delegates in Wisconsin raising a big question mark over the viability of his candidacy given that he has only one state victory, his home state, in the primary election.
The result in the Badger State raises the prospect of a showdown at the party's gathering in Cleveland from July 18th to 21st and the first multi-ballot convention to pick a Republican nominee since Thomas Dewey was chosen to face sitting president Harry Truman in the 1948 presidential election.
An open convention would mean that the Republican nominee would be chosen following a “floor fight” for delegates between the candidates.
Mr Trump needs to win 62 per cent of the delegates at stake in the remaining races, a task that is likely beyond him given that he has won just 46 per cent of delegates in the 40 state and US territory ballots held so far.
On the Democratic side, Mr Sanders insisted that he could still navigate a course to the party’s nomination, highlighting the recent momentum he has built up against the party’s one-time presumptive nominee, Ms Clinton.
"With our victory here tonight in Wisconsin we have won seven out of the last eight caucuses and primaries, and we have won almost all of them with overwhelming landslide numbers," he told supporters at his victory rally in Wyoming, the next state to vote in the Democratic caucuses on Saturday.
Mr Sanders beat Mrs Clinton in Wisconsin, another mostly white state where he has performed better than the second-time presidential candidate.
The 74-year-old senator won the young vote again overwhelmingly, winning the support of 18- to 29-year-old voters by a whopping 82 per cent to 18 per cent, according to exit polls from the Wisconsin voting precincts.
The democratic socialist from Vermont benefited from the midwestern state being an open primary where independents are free to vote and made up a quarter of Democratic voters; he polls well with independent voters.
Ms Clinton made no public statement or appearance following her defeat in Wisconsin.
Mr Sanders heads to the next big-state contest, the primary in New York - where he was born and where Ms Clinton lives and served as a US senator - requiring a big victory if he has any hope of closing the gap on her 200-plus lead among delegates given how they are awarded proportionately.
Polls show Mr Trump leading Mr Cruz in his home state of New York by an average of 32 points, according to election tracking website, RealClearPolitics, while Ms Clinton has a 11-point lead over Mr Sanders.