President Donald Trump launched his "Evangelicals for Trump" coalition on Friday night as he defended the US killing of Iran's top military leader, Qasem Soleimani, in the face of strong criticism.

A rally to mark the launch at El Ray Jesus Church in Miami was attended by high profile evangelical supporters, including James Dobson, Alveda King, Franklin Graham's daughter Cissie Graham Lynch, and his spiritual adviser Paula White. 

Addressing the crowds, Trump said he was confident that the evangelical turnout for his 2020 re-election bid was going to be even higher than during his election in 2016.

"In 2016, evangelical Christians went out and they worked so hard and they produced numbers like they have never produced before," Trump said.

"Based on what Paula told me and Cissie told me and everybody told me, we are going to blow those numbers away in 2020."

He defended his track record, saying that he had "achieved results nobody thought was possible."

"Evangelicals and Christians of every denomination and believers of every faith have never had a greater champion ... in the White House than you have right now," Trump said.

"I am not saying that in any other way other than just look at the record. Together, we are not only defending our constitutional rights, but we are also defending religion itself, which is under siege."

He told the rally that they must not allow someone from the "radical left" to take office. 

"Because everything we have done will be gone in short order," he said. "It will be taken away pretty quickly. It's a powerful position."

He went on to say that faith values were not "outdated" but "needed now more than ever", in addition to policies supporting the family. 

He said it was an "eternal truth" that faith and family are the key to the "stability, happiness and prosperity of nations". 

"There are those who say these sacred beliefs are outdated. But we know they are just the opposite," Trump said.

"Our traditions and our values are timeless and immortal. [The political left] don't know what they are missing. Our faith is needed now more than ever.

"Yet as we speak, every Democrat candidate running for president is trying to punish religious believers and silence our churches and our pastors.

"Our opponents want to shut out God from the public square so they can impose their extreme anti-religious and socialist agenda on America."

He continued: "This election is about the survival of our nation," he said. "With your help, your prayers and tireless effort to mobilise Christian communities across our land, on Nov 3, 2020 [...] we are going to win another monumental victory for faith and family, God and country, flag and freedom.

"I really do believe we have God on our side. There would have been no other way we could have won [in 2016], right?

"People said, 'You don't have the media, you have so many things against you.' But we win. So it has to be something."

The launch of the coalition came on the same day as a US drone strike killed Soleimani, rapidly escalating tensions in the Middle East and giving rise to fears of war after Iran promised "forceful revenge".

On Saturday night, Trump tweeted that Iran "will be hit very fast and very hard" if it retaliates.  Earlier he said that the strike had ended Soleimani's "reign of terror" and that it had been carried out "to stop a war, not to start one".

Evangelical leaders have defended Trump in the midst of the backlash. 

Franklin Graham said in a Facebook post that he was praying for Trump and US troops deployed to the Middle East.

"Yesterday the United States took out one of Iran's leading terrorists at the Baghdad airport," he said.

"Qassem Soleimani was responsible for countless deaths inside and outside Iran and was actively planning more terrorist attacks.

"We need to pray that God would give President Donald J. Trump wisdom as he continues the war on terrorism.

"Join me in praying that the President, his family, and our troops serving in the region would be protected from evil." 

In a separate post, he added:  "I agree with the President that we must defend the interests of our nation."