WASHINGTON — President Donald Trump vowed to stand with anti-abortion activists Friday as he became the first sitting president to speak at the March for Life, an annual gathering that is one of the movement’s highest profile and most symbolic events.

“Today as President of the United States, I am truly proud to stand with you,” he told a crowd of thousands braving the cold on the National Mall. “Unborn children have never had a stronger defender in the White House.”

It was just four years ago when a political committee supporting one of Trump’s Republican rivals unveiled an ad slamming his views on abortion, complete with footage from a 1999 interview in which he declared, “I am pro-choice in every respect.”

But on Friday, Trump was hailed in speeches and on signs as “the most pro-life” American president ever.

The reception was yet another sign of his remarkable political transformation and the fact that white evangelical and conservative Christians remain among Trump’s most loyal backers.

“I think it’s a brilliant move,” said Ralph Reed, chair of the Faith and Freedom Coalition and one of Trump’s most prominent evangelical supporters, of Trump’s decision to become the first president to take the event’s stage. Reed said the president’s appearance would “ energize and remind pro-life voters what a great friend this president and administration has been.”

It also shows how much times have changed.

Past presidents who opposed abortion, including Ronald Reagan and George W. Bush, sent remarks for others to deliver, spoke via telephone hookup or invited organizers to the White House — but never appeared at the march.

Over the last 10 years, the Republican Party has undergone a “revolution,” displaying a new willingness to “embrace the issue as not only being morally right but politically smart,” said Mallory Quigley, a spokeswoman for the Susan B. Anthony List and Women Speak Out PAC, which is planning to spend $52 million this cycle to help elect candidates opposed to abortion.

While views of abortion have remained relatively stable over two decades of polling — with roughly 6 in 10 Americans saying abortion should be legal in all or most cases, according to Pew Research Center — both parties have taken harder-line positions for and against.

“There used to be a middle in this country and candidates would not want to alienate the middle,” said Ari Fleischer, who served as White House press secretary under President George W. Bush. “And it just seems that that is over and that both parties play to their bases.”

During his first three years in office, Trump has embraced socially conservative policies, particularly on abortion. He’s appointed judges who oppose it, cut taxpayer funding and painted Democrats who support abortion rights as extreme in their views. On Friday, his administration took another step, threatening California with the potential loss of federal health care funding over the state’s requirement that insurance plans cover abortions.