After coming under fire for his earlier comments, US presidential hopeful Donald Trump says abortion laws shouldn't be changed.

It's the latest shift for the Republican front-runner on an issue that's caused him grief since he said earlier in the week there should be "some form of punishment" for women who got abortions if the procedure was outlawed.

He backed off that remark under fierce criticism, saying if abortion were no longer legal, those who performed abortions should be penalised, not the women who had them.

Meanwhile, Trump is facing pressure to take a more serious approach to his presidential campaign from supporters worried that a string of recent missteps and controversial comments may do lasting damage.

The probability that Trump will win his party's presidential nomination has dropped sharply in the past week while the likelihood of a brokered convention to potentially choose another candidate rose, according to online predictions market PredictIt.

Trump's probability of winning the nomination fell to 44 per cent on Friday from 67 per cent a week ago, according to the website, which is run by Victoria University in Wellington. For Republican rival Ted Cruz, a US Senator from Texas, it rose to 34 per cent from 14 per cent during the same period.

A Cruz win in Tuesday's primary in Wisconsin would make it harder for Trump to reach the 1237 delegates needed to clinch the nomination before the Republican national convention in July.

If Trump does not win enough delegates, the party faces the prospect of holding a contested, or brokered, convention in which party leaders try to negotiate nominating a compromise candidate.

Both Republican elders, as well as President Barack Obama, are urging Trump to adopt a more presidential tone in his run for the White House, with many of his Republican colleagues worried that his missteps may do irreparable harm to the party and his campaign.

The Republican front-runner also came under fire from Obama on Friday over recent comments that he would not rule out using nuclear weapons in Europe and that Japan and South Korea might need their own nuclear weapons.

Obama told a news conference that it shows Trump doesn't know much about foreign policy or nuclear policy or the Korean peninsula.