The two presidential candidates who emerged on top Tuesday in the Wisconsin primary claimed their campaigns are unifying their respective parties and gathering steam.

Republican Ted Cruz and Democrat Bernie Sanders were the projected winners. Each won with wide margins, and each said the wins showed their campaigns have the momentum needed to steal the nomination from their party's current front-runners.

The two US senators had been favoured to finish on top ahead of Republican Donald Trump and Democrat Hillary Clinton in the north-central US state.

Cruz, who represents Texas in the US Senate, had 49 per cent of the vote with 77 per cent of the ballots counted; Trump had 34 per cent and the third Republican candidate, Ohio Governor John Kasich, had 14, according to unofficial results reported by US media.

Sanders, who represents Vermont in the Senate, was leading Clinton 56 per cent to 44 per cent with 80 per cent of the votes counted.

Cruz who said his campaign for US president has racked up recent wins because he is uniting the Republican Party.

"Tonight is about unity and tonight is about hope," Cruz said, speaking to supporters in Milwaukee, Wisconsin.

Cruz, who has won four states in a row but still lags behind Trump in the delegate count, said Wisconsin voters had "lit a candle guiding the way forwards. Tonight we once again have hope for the future."

Many Republican party leaders have openly expressed their hopes for a game changer in the race that will derail the tough-talking Trump, whose controversial statements have offended immigrants, women and Muslims.

Observers said it was significant that Cruz was able to pull off a win in the US heartland where the New York real estate mogul has a perceived edge because of his stance on illegal immigration and trade pacts.

The win also came in a state with a large contingent of voters who are working-class and live in rural areas - just the type of people who have backed Trump in other states.

The question remains whether the Republican Party will head to its convention in July without a candidate who has the required number of delegates to claim the nomination outright. Cruz's victory in Wisconsin increased the likelihood of that scenario, observers said.

Sanders' defeat of Clinton, the former US secretary of state, US senator and former first lady, gives the senator his sixth win in a row in the state-by-state voting, but he still lags far behind Clinton in the delegate count.

The self-described democratic socialist, who rails about the income gap, the influence of Wall Street on politics and issues such as minimum wage and the cost of higher education, has now finished ahead of Clinton in six of the seven contests dating back to March 22.

Sanders, who spoke in Wyoming, where Democrats will caucus on Saturday, said his campaign is "giving energy and enthusiasm to millions of Americans."

Sanders urged supporters to stand together and continue their fight to defeat Clinton. He also touted his campaign's grassroots fund-raising and refusal to accept large donations from wealthy people and corporations.

"Yes we can create a government that represents all of us and not just a handful of wealthy campaign contributors," he said. "I think the people of this country are ready for a political revolution."

The state-by-state voting process in the US presidential race is about two-thirds complete. The next large states to hold a primary is New York on April 19.

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