Washington DC - Mike Pence demanded Friday that every Republican presidential hopeful back a nationwide abortion ban at 15 weeks or earlier as he sought to outflank Donald Trump on an issue his former boss has been accused of fudging.

Touting his pro-life credentials a year after the Supreme Court overturned the federally-protected guarantee of abortion access, the ex-vice president threw down the gauntlet to his rivals.

"The fact is today abortion law in the United States is more aligned with China and North Korea than with Western nations in Europe," he told the Road to Majority Conference, a gathering of 3,000 evangelical conservatives in Washington.

"So I want to say from my heart, every Republican candidate for president should support a ban on abortion before 15 weeks as a minimum nationwide standard."

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Pence didn't mention the former president by name, but his address was a pointed challenge to Trump, who views the religious right as key to his 2016 presidential win and future White House ambitions.

The former president thrilled conservatives when he appointed three justices to the Supreme Court during a single term in office who subsequently voted to strike down Roe v. Wade.

But anti-abortion activists have been dismayed by Trump refusing to publicly back a nationwide ban this time around even as he takes credit for the court's ruling.

Trump has also warned against tacking too far to the right and suggested that anti-abortion hardliners were to blame for the poor Republican showing in last year's midterm elections.

"Now some you will hear from at this very podium will say that the Supreme Court returned to the issue of abortion only to the states, that nothing could be done at the federal level," Pence said, addressing Trump's criticisms directly.

"Others will say that continuing the fight to life could produce state legislation is too harsh. Some have even gone on to blame the overturning of Roe vs. Wade for election losses in 2022."

"But let me say from my heart, the cause of life is the calling of our time and we must not rest and must not relent until we restore the sanctity of life to the center of American law in every state in this country."

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The Road to Majority conference – organized by the Georgia-based, two-million-member Faith and Freedom Coalition – brings together all the major Republican presidential hopefuls on the same stage for the first time.

Trump, who leads Florida governor Ron DeSantis by more than 30 polling points in the Republican primary, is due to deliver the keynote address at the closing gala on Saturday.

DeSantis speaks later Friday and is expected to talk up the passage of Florida's six-week abortion ban – a measure that Trump described as "too harsh."