When Vice President Pence late Wednesday morning addresses one of the country’s biggest Christian gatherings — the Southern Baptist Convention’s annual meeting — he’ll be following a decades-long line of White House Republicans who have come to speak to the right-leaning group.

Which is why experts on conservative Christianity were wowed by the sight Tuesday of multiple Southern Baptist pastors trying — through the meeting’s formal procedures — to block Pence’s talk or, at least, pass a ban on inviting politicians to future annual meetings. Video of the Dallas convention hall showed many hundreds of hands holding yellow ballots go up when a Virginia pastor argued that hosting a Trump administration official hurts Southern Baptists of color and endangers soul-saving in general.

None of the four separate measures passed (a few were referred for consideration in the coming year). But historians say the effort was the first real controversy in the convention about a GOP speaker since the evangelist Billy Graham pushed for the invitation of President Richard M. Nixon in 1972 and reveals the significant upheaval among conservative evangelicals about President Trump and the mixing of partisan politics and religion.

“For 35 years you could expect the Southern Baptist Convention to be pro-Republican in a nearly unanimous way. But 2016 means the relationship with the Republican Party for the Southern Baptist Convention has become problematic,” said Thomas Kidd, a history professor at Baylor University who has written books about American evangelicalism.

The convention is a major force in conservative evangelicalism and represents the largest Protestant denomination in the country.

Specifically what this development means, Kidd said, “is hard to say. But it signals that business as usual as far as [giving a platform to] Republican politicians — there will be pushback against that in a way there wasn’t.”

The question is whether the divide within evangelicalism will lead to different religious affiliation patterns, or different voting patterns, or something else — or nothing.

“What makes this unique is the amount of turmoil around the present administration, which has heightened all the fault lines so many of us feel, around racial reconciliation, and clarity about what Christians are about, which is Jesus and him dying for sinners,” said Garrett Kell, of Del Ray Baptist Church in Alexandria, Va.

Kell proposed a measure Tuesday morning that didn’t pass that would have replaced Pence on the agenda with a time of prayer. No vote count was taken, but many in the convention hall estimated that 30 to 40 percent of attendees had voted for Kell’s measure.

“For many years we have been talking about loving and listening to our minority brothers and sisters. This invitation does nothing to suggest that we are actually listening,” Kell’s measure reads. It also talked about the need for “clarity of the gospel” and for protecting the reputation of Southern Baptists.

“What binds this convention together is the Gospel of Jesus Christ. Because of that, this convention ought to be marked by things we share in common, not things about which faithful Christians can disagree,” he wrote. “We must do all we can to preserve the purity of the Gospel, and this invitation works against it.”

Polls have shown for years that one of the primary reasons young American Christians cite for leaving evangelicalism is because of their perception that the church has become overly politicized — and owned by the GOP. But experts said the scene of as many of a third of Southern Baptist delegates waving yellow voting cards against the vice president has a lot to do with Trump specifically. Pence, who grew up Catholic and later joined nondenominational evangelical churches, would likely not have been controversial on his own, they said.

With most Southern Baptists at the convention in support of Pence’s brief address — which reportedly was initiated by the vice president’s office — no further measures about him were expected to appear this week. However, some said they would skip Pence’s address and instead hold prayer sessions elsewhere in the building.

A fifth pastor Tuesday put forward a measure to replace Pence’s speech with a sermon by black Southern Baptist pastor H.B. Charles Jr. of Jacksonville, Fla. That motion was ruled out of order.

The split about Pence, like many splits among conservative evangelicals and in the convention, is pretty generational. It has to do with evangelical concepts of patriotism, America and, therefore, high government officials, said Trevin Wax, director of Bibles and reference at LifeWay, the publishing and research arm of the Southern Baptist Convention.

Older Southern Baptists see the U.S. as “Israel,” Wax said, a land with which God has a special relationship. Younger ones see America as “Babylon” — a place of potential corruption, a land where conservative Christians “should expect to be a minority, morally,” he said, and should focus on pastoral strategies and needs for what they see as cultural problems rather than fixating on political attempts to turn back that challenge.

In other words, younger Southern Baptists would see their role as a prophetic minority who can reach out to everyone, not partisan political power players, Wax said.

“It would be overstating it if we were to say there has been a wholesale change in regard to Southern Baptist political views from the older to the younger. What we’re seeing primarily is a change in posture related to our engagement,” he said. “Most of our divides aren’t theological but cultural. The posture is where the story is. That’s where the fault line is.”

Gerald Ford was the first U.S. president to speak to the Southern Baptist Convention meeting, in 1976, said Brian Kaylor, a Baptist pastor with a PhD in political communications who writes books on religion and politics. That was an era before conservatives had taken control of the convention — the country’s largest Protestant group — and the appearance of a Republican president was an effort to provide political balance. Many of the convention leaders at the time were close to Jimmy Carter — who was a Southern Baptist.

Since then, among the GOP White House officials to speak, Kaylor said, were George H.W. and George W. Bush, Condoleezza Rice and Dan Quayle. Ronald Reagan spoke to a key meeting of Southern Baptists and other evangelicals in 1980, telling them, famously, “I know you can’t endorse me, but I endorse you.”

The official initiation of partisan speechmaking to the Southern Baptists came from a man many considered above politicking: Billy Graham. But historian Kidd said Graham had pushed to invite Nixon, with whom he was very close, as far back as the 1950s, to speak with Southern Baptists because he felt it was essential for white evangelicals to come to the GOP over the issue of communism. Nixon declined the invite to the convention, Kidd said, because of the controversy about a political speaker.

Kaylor said the convention never heard from White House officials who were Southern Baptist — because they were Democrats, including Carter, Al Gore and Bill Clinton.

Nate Templin, an attendee from Colorado, said he believes the discomfort among some young attendees about Pence’s invitation indicates a desire to move away from a more traditional image of Southern Baptists — older, white, conservative, Republican.

“The younger generation, they’re saying it doesn’t matter which side of the aisle you stand on if you love Jesus and you love people,” Templin told The Post.

However, the defeated measure to block Pence reflected that many Southern Baptists were fully supportive of the vice president speaking at the gathering.

“I wanted to tell that guy to shut up,” Lillian Bohannan said after listening to Kell’s motion to have a time of prayer instead of Pence’s speech. Bohannan, 73, was attending her first Southern Baptist Convention meeting.

“Mr. Pence is a good guy. He was invited to speak as a Christian to a Christian group. We’re supposed to respect those in authority over us. That’s one way to do it,” she said.

Others said they believed those opposed to Pence speaking were more concerned about the image of Southern Baptists being tied to the Trump administration than about the invitation to the vice president.

“We live in a culture, we live in a day, where everybody’s hypersensitive about everything,” said Joe Donahue of First Baptist Church in Lavaca, Ark. “I think you would find probably 99.9 percent of people here support the president. Just as we supported Obama: through prayer.”

Julie Zauzmer contributed to this report.