US President Donald Trump will hold a campaign event in a Miami, Florida, megachurch on Friday to shore up support from Christian conservatives after a prominent evangelical publication questioned whether the faithful should support the Republican.

The event by Trump at the 7,000-capacity King Jesus International Ministry church has drawn fresh attention to his administration's ties to "prosperity gospel" preachers who tell followers that generous donations to their churches will be rewarded on Earth with wealth, health and power.

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The doctrine's growth in recent decades - often helped by charismatic televangelist pastors - has confounded classical theologians and some of the evangelical community who consider the "prosperity gospel" to be in direct opposition to the Bible.

The King Jesus International Ministry is led by Pastor Guillermo Maldonado, who encourages worshippers to give "first fruits" donations to the church in January that guarantee spiritual and financial returns later in the year.

"First fruits are given to honour God," Maldonado said in a typical Facebook message on his page. "You can't have the Father's favour until you honor Him."

The Trump administration has aligned itself with other prosperity gospel leaders, including Paula White, who has appeared at White House prayer events and was named last year as an adviser in the Executive Office of Public Liaison, which is tasked with outreach to special interest groups.

White and Maldonado's ministries did not respond to questions about criticism of the prosperity gospel practice.

The late Reverand Billy Graham denounced the prosperity gospel movement in 2016, saying: "Jesus wasn't rich, nor were His first disciples - not at all. In fact, the only disciple who really cared about money was Judas."

More than 80 percent of white evangelicals voted for Trump in the 2016 election. But a crack in evangelical support opened up last month when the magazine Christianity Today wrote a blistering editorial on Trump's "grossly immoral character".

Republican presidential candidate Donald Trump stands during a service at the International Church of Las Vegas in Las Vegas [File: Evan Vucci/AP Photo]

Researchers say it is hard to quantify the exact numbers of Americans who follow the prosperity gospel, but their numbers may be in the tens of millions.

According to a 2019 survey by the Pew Research Center, 43 percent of US adults, or some 110 million people, identify with Protestantism; 59 percent of those, or 64 million are born-again or evangelical Christians.

Many may follow prosperity gospel preachers on TV or online while identifying as mainline Protestants or evangelicals in surveys, academics say. Joel Osteen, head of a Houston megachurch, who is one of the country's most popular prosperity gospel preachers, says he reaches 100 million homes in the US through broadcasts, videos, and podcasts.

'Evangelicals for Trump' coalition

Trump's speech on Friday will mark the launch of the "Evangelicals for Trump" coalition ahead of the presidential election in November.

Last Sunday, Maldonado urged his congregation to attend the Trump event saying: "If you want to come, do it for your pastor. That's a way of supporting me," the Miami Herald reported on December 29.

Those comments appear to have violated tax rules barring religious groups from participating in political campaigns, the Freedom From Religion Foundation, a non-partisan group that advocates for the separation of church and state, said on Tuesday.

The church says it is not endorsing Trump.

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"The January 3 Evangelicals for Trump event is being paid for and organised by President Trump's election campaign. We agreed to lease space in exchange for fair compensation. No church resources are being used and our agreement to provide rental space is not an endorsement of President Trump's campaign or any political party," it said in a statement.

While the prosperity gospel is criticised among mainstream Christian denominations, "its basic tenets have crept into a remarkable proportion of Americans' theological world view," said Robert P Jones, chief executive and founder of the Public Religion Research Institute.

Paul Djupe, a scholar in the department of political science at Denison University in Ohio, conducted a nationwide survey with colleagues in 2018 that found about 32 percent of American adults agreed or strongly agreed with the statement: "God will reward the faithful with health and wealth."

Ryan Burge, a political scientist at Eastern Illinois University, has analysed 2012 survey data to show that Americans from low-income households were more likely to believe that God rewards faith and generosity with prosperity. The beliefs of the prosperity gospel were also more common among black Protestants.

"It makes poor people feel like they can be rich someday," said Burge, who is also a pastor in the American Baptist Church.

Pastor Robert Jeffress of First Dallas Baptist Church, who will deliver the benediction at Friday's Miami event, said he continued to back Trump because of his record of opposing the abortion rights movement, supporting Israel and nominating conservative judges.

While First Baptist repudiates the prosperity gospel, "it's not unusual that different flavors of evangelicalism would come together" to support a common cause, Jeffress said.

Trump's speech on Friday is also the first scheduled public appearance for the president since he ordered the killing of Iranian military commander Qassem Soleimani.