In his 2000 book The America We Deserve, Mr Trump wrote that he supported a woman's right to choose, but in 2011 he said he had changed his mind
Republican presidential candidate Donald Trump has said that if abortions are made illegal in the US women who have them should be punished.
"There has to be some form of punishment," he said in a fierce exchange at a town-hall style event in Wisconsin on Wednesday.
Pressed by MSNBC's Chris Matthews on the nature of that punishment, Mr Trump responded: "I haven't determined what the punishment should be."
The property billionaire's comments provoked a torrent of condemnation.
Democratic White House front runner Hillary Clinton pounced on Twitter, saying: "Just when you thought it couldn't get worse. Horrific and telling."
In the Wisconsin town hall, Mr Trump described himself as "pro-life with three exceptions", without specifying those clauses.
His campaign later issued a statement saying the abortion issue should be left to individual US states to determine.
US abortion rights were enshrined in the Supreme Court's landmark 1973 ruling in Roe v Wade.
However, a number of states have passed measures over the years limiting access to pregnancy terminations.
And criticising Roe v Wade has become a matter of conservative orthodoxy for any Republican who runs for president.
Mr Trump has previously been criticised by his Republican rivals who say he is not conservative enough on abortion.
The real estate magnate's views on the hot-button issue have evolved.
In his 2000 book The America We Deserve, Mr Trump wrote that he supported a woman's right to choose.
But in 2011 he said he had changed his mind.
Mr Trump more recently said he believes abortion should be banned at some point in pregnancy, with exceptions for rape, incest or life of the mother.
He arrived in Wisconsin defending his campaign manager over an alleged assault.
Corey Lewandowski was charged with misdemeanour battery in Florida on Tuesday over an altercation with a female reporter this month.
Wisconsin's 5 April primary could be a game-changer in the Republican race if Texas Senator Ted Cruz beats Mr Trump.
It could raise the prospect of the Republican nomination being decided by a contested convention, an extremely rare outcome that might plunge the party into civil war.
All three Republican candidates now say they aren't committing to supporting whomever the party chooses as its nominee for November's election.
Mr Trump said on Tuesday he was reneging on his promise because "I have been treated very unfairly" by the party establishment, which has shunned him.