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WASHINGTON — For the first time in years, abortion opponents will have all the political momentum when they hold their annual rally Friday on the National Mall.

The March for Life, held each year in Washington to mark the anniversary of the 1973 Supreme Court decision legalizing abortion, said it will have one of its biggest-name speakers in years: Vice President Mike Pence.

The March for Life said that neither a president nor a vice president has ever addressed the event now in its 44th year. And one of President Donald Trump's top advisers, Kellyanne Conway, is also on the speakers' list.

Organizers told the National Park Service in their permit application they expect 50,000 participants. Yet Trump insisted on the eve of the rally that the crowd would be far larger, saying "a lot of people are gonna be showing up."

Anti-abortion activists gather at the Washington Monument to hear Vice President Mike Pence speak at the March for Life rally on Jan. 27 in Washington Tasos Katopodis / AFP - Getty Images

"You know, the press never gives them the credit that they deserve," Trump told Republicans gathered in Philadelphia. "They'll have 300, 400, 500, 600 thousand people. You won't even read about it. When other people show up, you read big-time about it. Right? So, it's not fair, but nothing fair about the media."

A senior administration official told NBC News Friday morning that President Trump planned to call in remarks to the rally. Vice President Mike Pence and White House counselor Kellyanne Conway were expected to attend and speak as well.

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One of Trump's first official acts after taking office a week ago was to sign an executive order banning U.S. aid to foreign groups that provide abortions.

In Congress, Republican majorities in both chambers are vowing to end federal funding for Planned Parenthood, which provided more than a third of the nation's abortions in 2014. They also hope to ban most abortions after 20 weeks of pregnancy. Trump has pledged to sign both measures if they reach his desk.

Less than a year ago, with Barack Obama's second term winding down, things were markedly different. The Supreme Court struck down Texas' strict regulations on abortion clinics as interfering with a woman's constitutional right to terminate a pregnancy. And with polls at the time suggesting Hillary Clinton would likely defeat Trump, abortion opponents worried about an era of liberal majorities on the court.

Carole Seymour, right, of Shelton, Wash., carries a sign that reads "No More Killing" and features a sticker from the campaign of President Donald Trump as she takes part in an anti-abortion march and rally on Jan. 23, 2017, at the Capitol in Olympia, Wash. Ted S. Warren / AP

"The horizon looked bleak for the pro-life movement," said Jeanne Mancini, president of the March for Life.

Mancini suggested that many voters chose Trump largely because he pledged to appoint a Supreme Court justice who shared their views on abortion, even if they disagreed with him on other issues.

"I don't identify as a Republican or a Democrat but I do vote pro-life," Mancini said.

Abortion opponents also were heartened by a recent study that found the number of abortions in the United States dropped under 1 million in 2014, the lowest total in 40 years. The report by the Guttmacher Institute, which supports abortion rights, credited increased access to birth control but also a surge in abortion restrictions in many states.

Americans remain deeply divided on abortion.

The latest Gallup survey, released last spring, found that 47 percent of Americans described themselves as pro-choice and 46 percent as pro-life. It also found that 79 percent believed abortion should be legal in either some or all circumstances.

Pro-choice advocates (left) and anti-abortion advocates (right) rally outside of the Supreme Court on March 2, 2016 in Washington, DC. Drew Angerer / Getty Images, file

Ilyse Hogue, president of NARAL Pro-Choice America, said that poll shows why abortion-rights supporters shouldn't despair. She also said Republicans were taking actions that would result in more illegal abortions and deaths of pregnant women.

"The vast majority of Americans support Roe v. Wade and support the legal right to abortion," Hogue said.

Friday's march comes less than a week after one of the largest mass demonstrations in the city's history, the Women's March on Washington, which drew more than half a million people opposed to Trump on issues including abortion.

Although the landmark Supreme Court decision was Jan. 22, 1973, organizers of the march noted on their website that Trump was sworn in Jan. 20 and the National Park Service assigned Jan. 27 as the next available date for their event.

Mancini said she had planned to participate in the women's march until organizers dropped an anti-abortion group as an official partner. She said its failure to embrace different views on abortion was a missed opportunity.

The March for Life routinely draws thousands, even in harsh weather. Last year's was held in a blizzard that dumped nearly 2 feet of snow on the nation's capital.