Early exit polls from Wisconsin showed some good news for Democratic front-runner Hillary Clinton even though her rival was projected to win.

Just over half of Wisconsin Democrats say they want the next president to continue President Barack Obama's policies, according to preliminary exit poll data reported by NBC News.

That should help Clinton, who performs strongly among voters who want more of the same. Her rival, Senator Bernie Sanders does better among Obama's Democratic critics - including those who want more liberal policies and, oddly, those who think Obama was too liberal.

However Sanders was projected to win the Wisconsin Democratic primary contest, according to Fox News and NBC News. The last poll from Wisconsin last week showed Sanders widening his lead to 49 per cent support while Clinton had 45 per cent.

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There was also good news for Democrats generally.

More than 7 in 10 Democrats said they are "excited" or at least "optimistic" about Clinton and Sanders alike. No Republican candidate got such widespread positive reviews: The best was for Senator Ted Cruz, about whom 6 in 10 Wisconsin voters said they were excited or optimistic.

Barely 4 in 10 were excited about billionaire Donald Trump.

If Sanders could win here, it would give the "democratic socialist" from Vermont fresh dose of momentum - although, in truth, Sanders needs a huge win here to make up significant ground in the race for Democratic convention delegates.

In the Republican primary, most polls showed Cruz leading national GOP front-runner Trump by at least a few points. Cruz was projected to win.

Cruz gained significant support here in the past few days thanks to an endorsement by Wisconsin Governor Scott Walker and influential Wisconsin talk-radio hosts. He also owes some thanks to Trump, who has spent the time since the last GOP primaries inflicting a series of disasters on his own campaign.

Trump re-tweeted a fan who was insulting Cruz's wife. Trump called for some kind of "punishment" against women who have abortions. And Trump defended his campaign manager, who'd been charged with battery for grabbing a reporter, by suggesting that the reporter's pen could have been "a little bomb".

Now, Wisconsin could complete Cruz's remarkable turnabout - once the ultimate GOP pariah, he has become the last, best hope of the anti-Trump establishment. A new Reuters-Ipsos poll showed that Cruz is in a dead heat with Trump nationally after having trailed him by nearly 20 points a month ago.

If he wins in Wisconsin, Cruz will make it much harder for Trump to win the nomination without a fight at the convention. And if it comes down to a fight at the convention, Cruz has out-hustled Trump to secure support among individual delegates.

In early exit polling, only about one-third of Republican voters in Wisconsin said Trump had the best chance to beat Clinton in a general election, according to polls reported by ABC News. Cruz did better: More than 4 in 10 named Cruz as the Republican with the best chance. And fewer than 2 in 10 choose Ohio Governor John Kasich, the more moderate candidate running a distant third.

Trump admitted that he made missteps even as he refused to concede that polls showing him trailing Cruz were accurate.

"But I've had worse weeks on the campaign. I mean, I've had so many weeks that - I think a couple that were worse. And in one case I went up in the polls," he said. "So, you know, it couldn't have been so disastrous."

Trump has also dismissed speculation about a possible break in his momentum by pointing to his strengths in New York - his home state, which holds its primary on April 20 - and in the string of Eastern states that vote later this month.

"I've been all over Wisconsin. We're campaigning all over the place," Trump said in a Wisconsin diner.

"And I think we're going to have a surprise tonight. . . . I think we're going to have a great day," he added later. "I've worked hard in Wisconsin."

Anti-Trump Republicans, who have poured millions of dollars into attack ads around the country, are hopeful that a loss in Wisconsin would signal a break in the momentum that has kept Trump steadily rising in the polls.

A loss in Wisconsin, they believe, would increase the likelihood of a contested Republican convention in July - a strategy that rests on keeping Trump from crossing the requisite 1237-delegate threshold he needs to clinch the nomination outright.

In a private document circulated over the weekend and obtained by the Washington Post, Trump campaign senior adviser Barry Bennett revealed the mounting frustrations among the billionaire's top aides as they closed what had been a tumultuous week.

Titled "Digging through the Bull S**t," Bennett's memo urged Trump campaign manager Corey Lewandowski - who was charged with battery last week after allegedly yanking a reporter - and others to ignore critics who have questioned whether Trump's campaign has waned.

"America is sick of them. Their idiotic attacks just remind voters why they hate the Washington Establishment," Bennett wrote, citing tracking poll data favourable to Trump.

"Donald Trump 1," Bennett declared, as if he was scoring the past week. "Washington Establishment/Media 0."

That sort of sentiment resonated with Lisa Oleniczak, who voted for Trump at a precinct in Oak Creek, just south of Milwaukee.

"He's not from the establishment," she said.

A supporter gets a sign for Republican presidential candidate Senator Ted Cruz in Wisconsin. Photo / AP
A supporter gets a sign for Republican presidential candidate Senator Ted Cruz in Wisconsin. Photo / AP

But Ron Kurtz, 67, said he voted for Cruz because the New York billionaire's attacks on Senator John McCain and other elected GOP officials had gone too far.

"I don't like some of Trump's statements - like cutting down McCain for being captured," he said.

In the run-up to Tuesday, Clinton campaigned fewer days and before smaller crowds in Wisconsin than Sanders. She had already turned much of her attention ahead to the larger stakes in New York. She did not mention the Wisconsin race during a rally in Manhattan to cheer the state's approval of a US$15 minimum wage.

After holding a rally in Milwaukee, Sanders mingled with voters and stayed for breakfast at Blue's Egg diner, a city institution.

Forty-two delegates are at stake for Republicans, while 86 delegates will be awarded based on the Democratic vote (Wisconsin also has 10 Democratic "super-delegates," who can make up their own minds and aren't bound by the results of the vote). Since delegates are assigned proportionally on the Democratic side, Sanders will have a difficult time slicing into Clinton's overall lead if he does not win by an overwhelming margin.