Johnstown, Pennsylvania - Donald Trump tried Friday to contain the far-right blowback from his recent promise to ensure free access to in vitro fertilization (IVF) if he returns to the White House in November.

In explosive remarks on Thursday, Trump had also hit out at Florida's six-week abortion ban, calling it too restrictive and suggesting he planned to vote for an upcoming ballot measure that would make the procedure legal until a fetus becomes viable.

But at a rally in Pennsylvania on Friday, he backed down, telling Fox News: "I will be voting no."

Far-right Republicans had already begun criticizing Trump's ever-shifting positions on abortion, with a new party policy platform dropping calls for a national ban and the 78-year-old also claiming that his government would be "great" for reproductive rights.

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The pushback from extremist anti-abortion groups on his latest remarks was swift.

Evangelical theologian Albert Mohler said Trump's positions appeared "almost calculated to alienate pro-life voters" while conservative commentator Erick Erickson posted that Trump's abortion stance "will be a bridge too far for too many."

Trump's rally on Friday was notable for the absence of any remarks on reproductive rights, but Democrat Kamala Harris' campaign has been only too happy to weigh in on the issue.

"The majority of Americans support abortion access, they support IVF, they support contraception," Mini Timmaraju, of the Reproductive Freedom for All lobby group, told reporters in a campaign call.

"(Trump) has finally figured it out, and he'll do anything to distract from his abysmal, horrifying record on this issue."