The White House announced Wednesday that President Donald Trump will address the 45th March for Life, the nation’s largest annual pro-life event, via live stream this Friday.

Thousands of anti-abortion advocates are expected to flock to Washington, DC, for the annual rally held on the National Mall to galvanize public sentiment against abortion and formally advocate the overturning of Roe v. Wade.

The rally, which starts at 11:20 am and concludes at 3 pm, will include a performance of Christian music, a series of speakers — including House Speaker Paul Ryan and former NFL quarterback Tim Tebow -— and a march up Constitution Avenue between the Washington Monument and the Capitol.

Founded by anti-abortion activist and Roman Catholic convert Nellie Gray in 1974 to protest the one-year anniversary of the Supreme Court’s decision to legalize abortion nationwide in Roe v. Wade, the March for Life was intended to be a one-off event. After attracting 20,000 people that first year, it’s become an annual phenomenon with the support of religious advocacy groups, and plays host to several high-profile conservative and religious figures.

Though the march’s slogan, “a voice for the voiceless,” refers to what participants characterize as unborn children, it might well double for the participants themselves, many of whom see their values and their faith as at odds with an increasingly secular culture. Although Gray passed away in 2012, the March for Life organization continues to operate under president Jeanne Mancini, an author and Catholic activist.

Some years are better attended than others thanks to weather and political interest. In 2013, for example, just after the start of President Obama’s second term, Fox News reported a crowd of up to 650,000, up from 400,000 in 2012, while last year’s march — just after Trump’s inauguration — had numbers in the tens of thousands. In general, the march has been at its strongest as a countercultural force during periods when participants might be considered to be on the outs, politically: The best-attended year on record, according to the March for Life’s website, which does not give exact figures, was 2009, just a few days after President Obama’s inauguration.

It remains to be seen whether this year’s rally will be a protest or a victory lap. Trump’s presidency has galvanized evangelicals who, for the first time in a decade, see their interests represented at the White House. Last year, newly inaugurated Vice President Mike Pence became the highest-ranking member of a presidential administration ever to attend a rally in person. (Trump adviser Kellyanne Conway, a Catholic, also attended.) That year, Pence framed Trump’s victory as a major win for the anti-abortion movement, telling assembled crowds, “Life is winning again in America.”

The march draws its support from a religious base

While the march itself is not explicitly religious in nature, it is often suffused with Christian imagery, and its most high-profile participants tend to be religious leaders. Bodies like the Ethics & Religious Liberty Commission (ERLC), the public policy arm of the Southern Baptist Convention, and the Catholic Archdiocese of Washington (which organizes a related youth rally every year around the same time), have developed their own events pegged to or affiliated with the March.

Last year, for example, the march featured a prayer led by New York Catholic Archbishop Timothy Dolan. Expected speakers this year include the ERLC’s Russell Moore and Focus on the Family president Jim Daly.

Christian churches and organizations have also encouraged their members to participate. In a relatively uncommon step, the Archdiocese of Washington announced that a plenary indulgence — a total remission of punishment (i.e., purgatory) for sin — would be granted to those Catholics who attended masses related to the March for Life rally this year. (Plenary indulgences, a distinctly Catholic tradition that has become less fashionable in the modern era, tend to be reserved for more explicitly spiritual practices like pilgrimage or attending a spiritual retreat.)

The March for Life’s primary focus is abortion, but it’s also brought attention to anti-euthanasia and disability rights causes. In 2005, for example, the family of Terri Schiavo — whose final days on life support provoked a national debate on right-to-die policies — appeared on the March for Life stage.

Despite the beliefs of those converging in Washington on Friday, more Americans currently support abortion rights than ever before. According to the Pew Research Center, 57 percent of Americans support a woman’s right to an abortion in all or most cases. That said, it’s important to recognize that the connection between religion and abortion in America isn’t that clear-cut. Among Catholics — traditionally and historically abortion’s biggest opponents — 53 percent believe abortions should be legal in all or most cases, as do 55 percent of black Protestants. Only among white evangelical Protestants do we find a majority — 70 percent — that oppose abortion.

But among Protestants (if not Catholics), abortion has only become a hot-button issue in recent decades.

As Randall Balmer writes in Politico, as late as 1968 — just five years before Roe v. Wade — a medical symposium sponsored by the evangelical magazine Christianity Today declined to call abortion sinful, while throughout the 1970s (including after the passage of Roe v. Wade), delegates at the Southern Baptist Convention’s annual conference affirmed that “Southern Baptists [should] work for legislation that will allow the possibility of abortion under such conditions as rape, incest, clear evidence of severe fetal deformity, and carefully ascertained evidence of the likelihood of damage to the emotional, mental, and physical health of the mother.”

Rather, early evangelical political mobilization efforts centered not on Roe v. Wade but on Green v. Kennedy, the 1970 Supreme Court decision to strip private Christian segregated schools of their tax-exempt status, and the 1975 decision to do the same to evangelical Bob Jones University, which did not allow interracial dating among its students.

Balmer quotes Elmer L. Rumminger, an administrator at Bob Jones University, as saying that desegregation cases like these “alerted the Christian school community about what could happen with government interference [in evangelical institutions.] That was really the major issue that got us all involved.”

In fact, abortion did not prominently figure into the evangelical agenda until the 1978 election, when Jerry Falwell’s incipient Moral Majority used the topic to galvanize voters, many of whom resented perceived Democratic interference in the segregation cases.

As mentioned earlier, under Trump, evangelicals have seen more representation of their values in the White House, including taking stricter stances on abortion. During the presidential campaign, Trump said women should be punished for having abortions. Within days of taking office, he rolled back the global gag rule, barring foreign aid from contributing to abortions (though this rule typically changes with each incoming administration, depending on its political leanings). He and Republican lawmakers have also had no qualms with cutting funding for Planned Parenthood and other groups, and signed a law last year to officially cut federal support for the abortion provider.

Given all this, what he says to the crowd this year will no doubt be of note.