Donald Trump has promised to "totally destroy" the Johnson Amendment, fulfilling another campaign promise and prompting wild praise from Republicans and conservative Christians.

He told hundreds gathered for the National Prayer Breakfast on Thursday he would "get rid of and totally destroy the Johnson Amendment and allow our representatives of faith to speak freely and without fear of retribution".

Here are five points to help you understand the Johnson Amendment and why it is so vociferously opposed by many Republicans.

1. What is the Johnson Amendment?

Introduced in 1954 by then Senator Lyndon Johnson, it stops churches, charities and other organisations that are exempt from paying taxes from intervening in a political campaign.

It stops tax-exempt groups – including churches – from collecting money for a political campaign or openly endorsing any particular candidate from the pulpit.

The US' Internal Revenue Service explains that religious groups "are absolutely prohibited from directly or indirectly participating in, or intervening in, any political campaign on behalf of (or in opposition to) any candidate for elective public office".

2. Does this mean churches cannot be political in any way?

No. Pastors can preach on social issues and "issue guidance" to voters.

The law is actually very narrow in scope and is mainly centred around money. Churches can encourage voting, organise voter education events and preach on political issues, but just not endorse or openly support a particular candidate.

3. Isn't this violating free speech?

It is more about money and politics than it is about free speech.

Conservative groups who want religion to play a bigger role in public life have campaigned for the law to be overturned and claimed it inhibits free speech.

But it is actually one of the clearest barriers in the US between Church and state.

If the law were repealed and churches and faith leaders allowed to endorse and support candidates, religious support would become a huge factor in US politics.

If tax-exempt religious groups were allowed to donate large sums of money to political campaigns or candidates, churches could become vehicles for political fundraising.

4. Why do Republicans oppose the Johnson Amendment?

Because of their conservative status on social issues like abortion and gay marriage, Republicans tend to gain the majority of churches' support. This was shown in the 81 per cent of white evangelicals who backed Trump in the 2016 election.

So they have the most to gain financially by repealing the law.

But it was actually originally passed by a Republican congress and signed into law by a Republican president, Dwight Eisenhower.

5. Can Trump actually repeal the Johnson Amendment ?

Not on his own.

Despite his promise to "totally destroy" it, the Johnson Amendment is part of tax law and so will need to repealed by Congress.

This is not unlikely though, as the Republicans now control both the House and Senate.