The US president said mothers who had just given birth were being given the choice of keeping the child or allowing it to be killed.
The claim – which is demonstrably false – came as he spoke about late-term abortions.
“The baby is born, the mother meets with the doctor, they take care of the baby, they wrap the baby beautifully, and then the doctor and the mother determine whether or not they will execute the baby,” Mr Trump said to a chorus of boos.
He rounded on Wisconsin’s Democrat governor Tony Evers – who, earlier this year, vetoed a Republican bill that would have required doctors to provide medical care to babies born alive after failed abortion attempts.
Mr Evers said he did not support the bill because he believed existing laws offered enough protection to such babies.
The inaccuracy was one of a series of extraordinary claims made by the president during a typically bellicose rally in the city of Green Bay.
He implied that he had continued to deal with the Middle East country after its leaders are said to have ordered the execution of Washington Post journalist Jamal Khashoggi because they “have a lot of money”.
At one point he revelled in anti-Hillary Clinton chants of “lock her up”. At another, he said his proposed plan to send all undocumented immigrants to just a handful of America’s sanctuary cities had been “my sick idea”.
He also dismissed rumours that, if voted in for a second term in 2020, he would try to change the constitution so he could run for a third time.
“I promise at the end of six years, I’ll be very happy but you’re gonna be left with the strongest country you’ve ever had,” he said.
The rally, which Mr Trump said had attracted more than 10,000 people, was held as a counter event to the annual White House Correspondents’ Dinner in Washington, which is traditionally attended by sitting presidents.
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