Donald Trump and Ted Cruz are headed to a photo finish in Wisconsin next Tuesday according to the latest poll numbers, which show the Texas senator ahead of the New York billionaire by just a single percentage point.

Public Policy Polling, a left-leaning firm, surveyed likely voters in the Badger State and found Cruz leading the Republican primary contest with 38 per cent, compared with Trump's 37.

The numbers were released Thursday, just two days after a poll sponsored by the Marquette University Law School projected Cruz with a solid 40-30 lead.

The dramatically different results could be explained by sample sizes: Marquette's pollsters interviewed 471 likely Republican primary voters, while Public Policy Polling surveyed 768.

They could also reflect quickly shifting political winds in Wisconsin, seasoned with a series of Trump rallies that tend to create unpredictable news cycles.

NOT DEAD YET: Donald Trump is running neck-and-neck with Ted Cruz in Wisconsin among Republicans and might pick up crossover Democrats who don't show up in opinion surveys about GOP candidates

HOPING THE FIRST POLL WAS RIGHT: Cruz was up by 10 points just two days ago, and he might win if John Kasich's voters peel off and put him over the top

Buried in the numbers are contradictory trends – one that might help Cruz and another that could benefit Trump.

The reality TV star and real estate tycoon is likely to benefit from a sizable number of Democratic 'crossover' voters – middle-class whites, many in labor unions – who will support Trump more on personality than on policy.

That entire group is invisible in a poll that excludes Democrats, as both the Marquette and PPP surveys did.

Wisconsin's primary is an 'open' one, meaning the Republican contest is not limited to only registered Republican voters. Trump has performed markedly better in open-primary states than in those with closed elections.

On the other side of the coin, Ohio Gov. John Kasich registered 17 per cent in the PPP poll of Wisconsin Republicans, but just 63 per cent of that group said they were definitely going to vote for him on Tuesday.

In the group that wasn't so sure, 51 per cent said they could switch to Cruz. Just 19 said the same of Trump.

The result is a 49-41 drubbing – with the Texas tea party titan emerging on top – if Kasich weren't in the race.

But the Ohioan shows no sign of backing away from the White House contest, and his presence is giving Trump a wider lane to swim in.

The PPP poll found that the billionaire front-runner is the only one of the three men whose 'favorable' score among voters is lower than his 'unfavorable' number – a lopsided 45-48 showing.

That suggests Trump's 37 per cent poll number in the Tuesday's primary couldn't possibly rise higher than 45.

Such a number would be more than enough to win a three-man race where a distant straggler like Kasich commands just 17 per cent.

In a head-to-head contest, 45 per cent is a losing proposition.

Kasich's favorable-unfavorable numbers look better at 52-30, and Cruz's are 50-36. But if Cruz can't can overtake Trump, he'll miss out on many of the 42 delegates up for grabs on Tuesday.

Wisconsin will award 18 delegates to the statewide victor, and another 24 in three-delegate batches to the winners of the state's eight congressional districts.

It's likely that the top vote-getter will collect either 30 or 33 delegates, with the remaining 9 or 12 spread among the two losers.