A Republican strategist expressed surprise about President Donald Trump's relative lack of involvement in a key congressional race.

Republican Matt Van Epps is facing a surprisingly strong challenge from Democratic state Rep. Aftyn Behn in Tennessee's seventh congressional district, which is holding a special election Tuesday. The president won by more than 20 points last year, and GOP strategist Doug Heye told "CNN News Central" that Trump could have done more for the candidate he endorsed.

"If this is in single digits, it's a big warning sign for Republicans moving to the midterm elections," Heye said. "We do overrate this, but if it is that close, it's going to be a big deal, and one of the things when you get to Election Day that campaigns say to themselves, this is true, Republican or Democrat ... you ask yourself, did we leave everything on the field?"

Heye faulted the president's efforts and wondered why he had basically stayed on the sidelines.

"My fear for Republicans on this one is, they didn't leave everything on the field, and I say that not about Mike Johnson and the [National Republican Senatorial Committee]," Heye added. "They have left everything on the field. But Donald Trump should have gone on the ground. Donald Trump used to love doing political rallies. I'm surprised he didn't do it here. We saw him phone it in, but that's exactly what he did. He phoned it in."

"If Donald Trump had gone on the ground in Tennessee and done a big rally for this, he would have been able to claim credit, if Republicans win tonight, about how he brought this race home," Heye added.

"That, to me, is a surprise."

Host Kate Bolduan highlighted Heye's remark, pointing out that Trump made a phone call to an event rather than making a personal appearance, as he had in the past.

"Literally phoned it in – Doug really means it," Bolduan said. "He literally was put on speakerphone yesterday."

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