Ahead of his rally, Trump told the radio show "New Hampshire Today" he didn't think that Lewandowski had made a decision yet on whether to run, but praised him.
"I think Corey is a fantastic guy. And I don't think he's made that decision yet. I will say this, if he ran, he would be a great senator," Trump said. "If he ran and won, he'd be a great senator. He would be great for New Hampshire."
Lewandowski told ABC News earlier this month that he was "seriously considering" running for Senate, with hopes of unseating incumbent Democratic Sen. Jeanne Shaheen who is up for re-election next year.
"I'm seriously considering it," Lewandowski said. "Senator Shaheen has failed the people of New Hampshire by voting in lock step with Chuck Schumer and Nancy Pelosi. The people of New Hampshire deserve better. If I run, I would be a voice for all the people of New Hampshire."
In a confidential memo obtained by ABC News, veteran pollster Tony Fabrizio, who also conducts polls for the Trump campaign, found from a survey of 400 likely 2020 primary voters in New Hampshire, that Lewandowski was leading the current GOP field, though roughly half of those who responded were undecided.
"Corey Lewandowski would enter the race as the GOP front-runner -- leading the field by double digits due to his stronger personal ratings," the August 13 memo stated. "He could clearly see his lead expand even further with an endorsement from President Trump, who is extremely popular with these GOP voters."
The poll also included other Republican Senate candidates including Ret. Brig. Gen. Don Bolduc and New Hampshire state Rep. Bill O’Brien. Lewandowski had the highest unfavorability rating among those polled.
While the president hasn't formally endorsed him, he has retweeted Lewandowski's tweets of reports that he is leading the GOP Senate field in polls in New Hampshire.
The New Hampshire Democratic Party released a statement earlier this month calling him a "craven lobbyist who has been credibly accused of assault many times."
"Corey Lewandowski is a craven lobbyist who has been credibly accused of assault many times and is chomping at the bit to strip away Granite Staters' health care," New Hampshire Democratic Party spokesman Josh Marcus-Blank said. "Meanwhile, Senator Shaheen is making a difference for New Hampshire families, leading efforts in the Senate to expand access to health care and taking on the big drug companies to lower the costs of prescription drugs. The contrast couldn't be more clear."