US president Donald Trump called it his “profound honour” to be the first president to attend the annual anti-abortion gathering in Washington called the March for Life.
He used his speech to attack Democrats as embracing “radical and extreme positions” on abortion and praised those attending the event, saying “unborn children have never had a stronger defender in the White House”.
Mr Trump once declared in a 1999 interview that “I am pro-choice in every respect”.
On Friday, prompting loud cheers from the many thousands attending the march, Mr Trump said: “Every life brings love into this world. Every child brings joy to a family. Every person is worth protecting.”
Mr Trump is counting on the support of his base of conservative activists to help bring him across the finish line as he heads into the 2020 election.
Past presidents who opposed abortion, including Ronald Reagan and George W Bush, steered clear of personally attending the march.
They sent remarks for others to deliver, spoke via telephone or invited organisers to visit the White House.
Over the last 10 years, however, 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.
The group is planning to spend 52 million dollars this cycle to help elect candidates opposed to abortion rights.
Its president, Marjorie Dannenfelser, will serve as national co-chair of a new campaign coalition, “Pro-life Voices for Trump”.
Over time, though, both the Republican and Democratic parties have taken harder-line positions for and against abortion rights.
“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 to get maximum turnout from their base.”
In addition, Mr Flesicher said, Mr Trump is far less tethered to tradition than past presidents and “happy to go where his predecessors haven’t”.
“President Trump has done more for the pro-life community than any other president, so it is fitting that he would be the first president in history to attend the March for Life on the National Mall,” said White House spokesman Judd Deere.
Alexis McGill Johnson, acting president of the Planned Parenthood Action Fund, accused the president of carrying out “a full-out assault on our health and our rights”.
“While Trump stands with the small number of Americans who want politicians to interfere with their personal health decisions, we’ll be standing with the nearly 80% of Americans who support abortion access,” she said.
The first march took place on the west steps of the Capitol in January 1974, the year after the Supreme Court decided Roe v Wade, the landmark ruling that established a woman’s legal right to abortion.