Donald Trump will become the first U.S. president to attend the annual "March for Life" to be held in Washington on Friday, organizers said, underscoring his outspoken support for the anti-abortion movement as it celebrates key legislative gains.Thousands of protesters from around the country were expected to converge in the nation's capital for the event, which began in 1973 after the U.S. Supreme Court, in its Roe v. Wade decision, established a woman's constitutional right to get an abortion."See you on Friday ... Big Crowd!" Trump posted on Twitter on Tuesday in response to a tweet from March for Life promoting the event.With the 2020 presidential campaign season heating up, abortion remains one of the most divisive issues in the United States. Opponents cite religious beliefs to declare it immoral, while abortion-rights activists say the procedure is protected by a constitutional guarantee that gives women control over their bodies and futures.About 58% of American adults say abortion should be legal in most or all cases, according to a Reuters/Ipsos poll last year.Even so, anti-abortion advocates made significant legislative strides in 2019. Twenty-five bans on various types of abortions were signed into law, according to the Guttmacher Institute, although many have not taken effect because of pending legal challenges.Conservative lawmakers have said some of the bans were passed with the knowledge that they likely would be struck down in court but with the hope that those rulings might prompt the Supreme Court to review its Roe v. Wade decision.In Roe v. Wade, the court found that certain state laws outlawing abortion were an unconstitutional violation of a woman’s right to privacy, effectively legalizing abortion nationwide.Even though he had declared support for abortion rights years earlier, Trump vowed during his 2016 campaign to appoint Supreme Court justices he believed would overturn Roe. Since his election, he has appointed two justices to the court, Neil Gorsuch and Brett Kavanaugh, cementing the court’s 5-4 conservative majority."You’ve heard a lot of religious leaders and a lot of Republicans say that this president is the biggest champion for life ... the biggest advocate for the pro-life movement in history," White House spokesman Hogan Gidley told reporters on Thursday.March For Life President Jeanne Mancini said the organization "was deeply honored" to welcome Trump in person, after he delivered televised remarks in support of the anti-abortion movement at the 2019 march. Vice President Mike Pence attended the event in person last year.Past U.S. presidents have opted to stay from the march. Republicans Ronald Reagan and George W. Bush both delivered remarks remotely.In June, the Supreme Court is expected to rule on a case that could drastically limit doctors' ability to provide abortions in Louisiana, a Republican stronghold state. The case will test the willingness of the court to uphold Republican-backed abortion restrictions being pursued in numerous conservative states.