Image copyright Alex Wong/Getty Images Image caption President Trump prays with faith leaders in the Oval Office

Evangelical support for US President Donald Trump is back in the spotlight after the resignation of a leading journalist for Christian Post magazine.

Journalist Napp Nazworth's departure follows an op-ed from another Christian outlet calling for Mr Trump's removal.

The ensuing outcry has served as a proxy war among US evangelists over Mr Trump's largely unchallenged grip on the religious right.

He has claimed overwhelming evangelical support since taking office.

So what is behind this conflict and what are the consequences for the president?

How did this controversy begin?

Last week, after the US House of Representatives voted to impeach Mr Trump, Christianity Today published an editorial by editor-in-chief Mark Galli urging the president's removal.

Deriding Mr Trump's "grossly immoral character", Mr Galli described the president's expulsion from office as a Christian imperative: "Not a matter of partisan loyalties but loyalty to the Creator of the Ten Commandments".

Media playback is unsupported on your device Media caption The church defying Trump by hiding immigrants facing deportation

Mr Trump "attempted to use his political power to coerce a foreign leader to harass and discredit one of the president's political opponents," Mr Galli wrote. "That is not only a violation of the Constitution; more importantly, it is profoundly immoral."

And the magazine - founded by one of the most influential preachers of the 20th Century, Billy Graham - went even further, pointing the finger at evangelicals who have remained devoted to the president "in spite of his blackened moral record".

"Remember who you are and whom you serve," Mr Galli wrote.

Why is this significant?

Since Mr Trump became president, he has laid claim to resounding support from evangelical Christians - bolstered by his selection of evangelical Mike Pence as his vice-president.

In the 2016 presidential election, 80% of self-identified white, born-again or evangelical Christians voted for Mr Trump, according to analysis by Pew Research Center.

Image copyright Drew Angerer/Getty Images Image caption Mr Trump's selection of evangelical Christian Mike Pence as vice-president has bolstered his support among the religious right

Mr Trump's success among evangelicals follows a political pattern in the US: in every presidential election since 2004, white, born-again and evangelical Christians have, on the whole, voted for the Republican nominee.

But the president's support among evangelicals cannot be explained merely by party affiliation. Indeed, the famously brazen US president managed to match or exceed the support among this demographic won by President George W. Bush in 2004, and presidential candidates John McCain in 2008 and Mitt Romney in 2012.

And since Mr Trump's move to the White House, he has kept a series of promises to his religious voters.

He nominated two reliable conservatives, Neil Gorsuch and Brett Kavanaugh, to the Supreme Court.

Add to that the loosening of government mandates that health-insurance plans include free contraceptive coverage.

Mr Trump also eased restrictions on political activities by religious organisations. And he increased restrictions on government support for international organisations that provided family planning and abortion counselling.

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And, in an apparent snub to what some see as an unofficial ban on the traditional Christmas greeting, the president has said: "We're saying Merry Christmas again".

Such promises have paid off. While support among some US Christians has dipped, an NPR-PBS NewsHour-Marist poll from earlier this month found 75% of white evangelical Christians approve of Mr Trump, compared with 42% of US adults overall.

This support is critical to Mr Trump: a victory in the 2020 election will rely on another win among social conservatives, evangelical Christians included.

What has the response been?

Christianity Today's searing editorial provoked a split reaction among American Christians - mirroring divisions among Christians in their support for the president.

Some followed the magazine's lead in breaking ties with the Republican president, while others doubled down on their support for Mr Trump.

Image copyright The Washington Post/Getty Images Image caption Mr Trump speaks before signing an executive order aimed at easing a tax rule limiting political activity for churches

On Sunday, almost 200 evangelical leaders and other supporters of the president, including former Arkansas governor and two-time Republican presidential candidate Mike Huckabee and former Minnesota Republican Michele Bachmann, penned a letter slamming Christianity Today.

"Your editorial offensively questioned the spiritual integrity and Christian witness of tens-of-millions of believers who take seriously their civic and moral obligations," they wrote. "It not only targeted our President; it also targeted those of us who support him, and have supported you."

The president also weighed, taking to Twitter on Friday to dismiss Christianity Today as a "far left magazine".

Skip Twitter post by @realDonaldTrump A far left magazine, or very “progressive,” as some would call it, which has been doing poorly and hasn’t been involved with the Billy Graham family for many years, Christianity Today, knows nothing about reading a perfect transcript of a routine phone call and would rather..... — Donald J. Trump (@realDonaldTrump) December 20, 2019 Report

But while the article lost the magazine a reported 2,000 subscriptions, it gained 5,000 new readers, drawn from a younger and more diverse audience, the Washington Post said.

And on Sunday, Christianity Today president Timothy Dalrymple defended the editorial, decrying the "enormous damage" wrought by "the alliance of American evangelicalism with this presidency".

"Christianity Today is theologically conservative. We are pro-life and pro-family," Mr Dalrymple wrote. But "the cost has been too high. American evangelicalism is not a Republican PAC".

What does Napp Nazworth have to do with it?

On Monday, journalist Napp Nazworth announced he was "forced to make the difficult decision to leave The Christian Post".

Image copyright Alex Wong/Getty Images

Mr Nazworth - a political editor and near 10-year veteran of the Christian magazine, whose Twitter biography includes the hashtag #NeverTrump - said the publication "decided to publish an editorial that positions them on Team Trump".

He continued: "I can't be an editor for a publication with that editorial voice."

Both Mr Nazworth's decision and Christianity Today's editorial suggest cracks in Mr Trump's evangelical stronghold.

But a Christian rush to the Democrats remains unlikely, especially as the parties remain divided on abortion rights, a key issue for social conservatives.

According to a study from Pew in October: while whites who identify as Christians still make up about two-thirds of all Republicans, they now amount to only one quarter of Democrats.