Mr. Trump, who once called himself “very pro-choice,” has until now addressed the group only remotely and welcomed some marchers at the White House.

But as he has battled for political survival in the face of multiple investigations and a re-election campaign, he has made increasingly warm overtures to evangelicals and sought to cast himself as the most firmly anti-abortion president in at least a generation.

His administration has already embraced the march, which draws thousands of attendees, in unprecedented ways. In 2017, Mike Pence became the first sitting vice president to attend the event, and the next year, Mr. Trump became the first president to address the rally by video. In his speech, the president vowed that his administration would “always defend the very first right in the Declaration of Independence, and that is the right to life.”

Many of the evangelical Christians who most strongly oppose abortion find Mr. Trump’s two divorces, penchant for profanity and lack of demonstrated interest in religion before he entered politics to be unsettling. But as a political bloc, they have stood by him loyally as he hands them long-sought judicial appointments and policy changes.

In a statement, Jeanne Mancini, the president of March for Life, praised the president for appointing anti-abortion judges, cutting taxpayer funding for abortions in the United States and abroad, and outspokenly opposing late-term abortions.