Predictions of a looming Democratic “blue wave” haven’t soured Christian conservative activists, who say President Trump has notched so many surprise wins that they have faith he will again defy the naysayers and deliver Republican victories in November.

Leaders on the religious right are more measured, saying Republican control of the House is very much in jeopardy and warning the Republican faithful not to become overconfident in Mr. Trump’s ability to deliver the impossible.

The midterms could be much closer than many expect, Tony Perkins, head of the Family Research Council, told reporters last week. “I think what you are going to see is a blue trickle,” he said. “I don’t think you are going to see a red wave.”

The activists at the Values Voter Summit, an annual FRC-organized gathering of Christian conservatives, were brimming with optimism after three days of meetings in Washington. They said Mr. Trump has been the ultimate warrior for pro-life and traditional values causes.

“I agree with Trump. I think we will see a red wave,” said John La Tour, a member of the City Council in Fayetteville, Arkansas. “I think Americans will not be fooled by what is going on, and they see the anger of the left and the so-called progressives. I think rational Americans will vote and less-rational Americans will stay home.”

That kind of faith in Mr. Trump from the religious right hasn’t always been a given.

In 2016, many of the movement’s leaders sought to unify behind another candidate to stop Mr. Trump, figuring both politically and issueswise that he was not their best horse to ride into the White House.

But they have been won over.

“What has surprised me is that he has been so gung-ho about conservative issues,” said Dawn Marie Barge of Maryland.

Asked whether she believed Mr. Trump’s “red wave” prediction, Mrs. Barge, 48, said, “Well, he has been right before, so we can only hope.”

Elections handicappers say those are vain hopes.

Polls show more voters plan to cast ballots for Democrats in their local House races than for Republicans. Also, analysts put dozens of Republican-held House seats at risk and consider only a handful of Democrat-held seats to be in play.

Mr. Trump, who points out that polls before the 2016 presidential election suggested he was going to lose, doubts all of the grim predictions.

“You know, it’s interesting: We have the strongest economy in the history of our nation, and I hear about a blue wave. I say, ‘Why is there a blue wave?’” Mr. Trump said a recent rally in Ohio. “We are doing better now with jobs. Today there are more people working than at any time in the history of our country, so I don’t think there’s going to be a blue wave. I hope there’s a red wave.”

Jim Gardner of Oklahoma said he believes Trump-led Republicans will defend their party’s majorities in Congress and that conservatives are excited for the president to stay the course.

“I’d grade him as an A or an A+,” the 67-year-old said. “I think most religious people, Christian people, are very surprised, but pleased because Trump doesn’t have that reputation as a religious man or Christian man, but he has done more to promote the causes that we are concerned about than any previous president.”

Richard Wagnon said people underestimate Mr. Trump’s electoral pull at their own peril. He said he and his wife prayed for Mr. Trump on the eve of the 2016 election when most believed the New York businessman was headed toward defeat.

“We looked at each other and said if he wins it is only because God intervened,” Mr. Wagnon said. “God can use people, whether they are perfect or they are not, to achieve the things that are good for us. So with that in mind, now you have all these people that are really angry, and I think there will be some [congressional races] that are close, obviously, but I think at the end of the day we will maintain the House and the Senate.”

As for Mr. Trump’s prediction, the 52-year-old businessman said the president says a lot of things.

“You know red wave? OK, I will go with that,” Mr. Wagnon said. “You know, I don’t think it is a blue wave.”

Ken DeVore of California said there is no predicting American voters.

“I will not predict blue or red,” said Mr. DeVore, adding that it is hard to count out Mr. Trump. “If anybody can pull it off, it is Trump.”