Washington (AFP) - US President Donald Trump, seeking to shore up evangelical support ahead of the November election, addressed the country's biggest annual anti-abortion rally on Friday as the Senate sat in judgment nearby at his impeachment trial.

"Unborn children have never had a stronger defender in the White House," Trump told tens of thousands of anti-abortion campaigners at the 47th "March for Life" on the National Mall.

Trump, the first president to address the annual event in person, assailed Democrats for supporting abortion rights and claimed that "religious liberty" was under attack.

"When it comes to abortion," the 73-year-old Republican president said, "Democrats have embraced the most radical and extreme positions."

"We have taken decisive action to protect religious liberty," he added.

Following Trump's brief remarks, the crowd broke into a chant of "four more years," a reference to the November vote, when Christian evangelical support will be crucial to Trump's re-election.

"I vote pro-life," said signs held by members of the crowd.

Trump made only a single reference to the impeachment trial going on just down the street that could see him removed from office for abuse of power and obstruction of Congress.

"They are coming after me because I am fighting for you and we are fighting for those who have no voice," he told the crowd. "And we will win because we know how to win."

Beverly Atkins, 55, drove 13 hours from Alabama to attend the rally.

"Life begins at conception," Atkins told AFP. "We don't think you should destroy that."

As for impeachment, she said Democrats "have no evidence" that Trump did anything wrong.

The seven Democratic impeachment "managers" are to make their final arguments at Trump's Senate trial on Friday.

White House lawyers will then have 24 hours, starting at 10:00 am (1500 GMT) on Saturday, to present their defense of the president, who was impeached by the House of Representatives on December 18.

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- 'I love Trump' -

Impeachment may forever stain his record, but Trump is virtually assured of acquittal by the Republican-held Senate -- and he is already looking past the trial to the re-election fight that awaits.

Before entering politics Trump defended abortion rights, but he has increasingly aligned himself with the anti-abortion movement as he works to firm up his electoral base.

"I love Trump," said Jim Bolognio, 63, who came to the rally from New Jersey. "These people are going to reelect him."

"My daughter is adopted," Bolognio added. "She wouldn't be here if her mother didn't choose the right path."

Brayden Harris, 16, who came to Washington from Hershey, Pennsylvania, with the "pro-life" club of his school, said "abortion is wrong."

"God says he knows you before you're even born, so in the womb he knows you," Harris said. "It's what it says in the Bible.

The "March for Life," which Trump had addressed by video the last two years, is held annually on or near the anniversary of the Supreme Court's ruling in the Roe v Wade case which legalized the procedure nationwide on January 22, 1973.

Under Trump's presidency, abortion rights activists fear that landmark ruling is now under threat.

"Since day one, this administration has carried out a full-out assault on our health and our rights," said Alexis McGill Johnson, acting president of the Planned Parenthood Action Fund, which supports abortion rights.

Since taking office in 2017, Trump has strengthened the Supreme Court's 5-4 conservative majority, naming two justices who oppose abortion -- Neil Gorsuch and Brett Kavanaugh -- and galvanizing abortion opponents.

Conservative-leaning Chief Justice John Roberts is seen as the potential swing vote if the constitutionality of abortion eventually comes before the court.

Around two-thirds of Americans say abortion should be legal, polls suggest.

But while Roe v Wade remains the law of the land, numerous states have taken measures to limit access to the procedure.

The first big Supreme Court test of that anti-abortion push will come in March, when the court examines a Louisiana law whose restrictions on abortion are similar to a Texas law struck down nearly four years ago.

Like each year, the march wound its way from the National Mall towards the Supreme Court near the Capitol, where senators are sitting as jurors in Trump's impeachment trial.

Republicans hold a 53 to 47 edge in the Senate and a two-thirds majority of the 100 senators would be needed to convict and remove the president from office.