The polls are open in Wisconsin, where cold weather did not stop people from lining up to vote this morning.
Huge numbers are expected to turn out to decide their choices in the Republican and Democratic primaries.
Donald Trump went to a diner in Waukesha to try to overcome a slight deficit in recent polls.
"They are all saying 'Trump, Trump, Trump.' There is something. It's going to be interesting tonight. It will be very interesting to see what happens. I think we are going to do great," Trump said..
For Democrats, a Wisconsin loss by Hillary Clinton could give Senator Bernie Sanders the boost he needs. As Sanders campaigns in Wisconsin, Clinton moved on to New York, where she's targeting the Republicans.
"They all want to take us back to trickle down economics, and we cannot allow that happen," Clinton said.
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