Veteran pollster Pat Caddell told SiriusXM host Alex Marlow on Monday’s Breitbart News Daily that Hillary Clinton’s health remains a major issue with voters.

“If you look at the questions about her health–you know, we have a Breitbart/Gravis poll–we had asked that question in our national survey this weekend.” He added, “The Fox polling showed on that question, in late August, that 21 percent of the people had concerns about her health. Well, it’s now over 40 percent,” Caddell revealed.

He said:

That’s still a lingering and lurking element here, and it really puts a lot of pressure, I’ve been saying, for her in this debate. People are so sensitive to it, and it may not be fair, but if she has moments like that, it could affect the audience and how they feel about her because of their own doubts on that question. So it could be a problem.

By “moments like that,” Caddell was referring to a much-discussed video of Clinton appearing to lose control of her voice, shouting angrily that she should be “50 points ahead” instead of locked in a dead heat with Donald Trump.

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Caddell agreed with Marlow that a health crisis for Clinton during one of the debates could “seal the deal” for Trump, noting that everyone in the media has “very nicely put that [issue] away, but the voters haven’t.”

“They have concerns over whether she can do the job as president,” said Caddell. “Ninety minutes is a long, long time without a break. You have to be concerned about stamina, particularly under those lights, under the pressure. It’s very hard. And so that is an aspect we need to keep in mind as we see the debate tonight.”

“Hillary Clinton’s lead with millennials has almost evaporated – 29 points, now down to 10 points. What do you make of Hillary’s millennial problem?” Marlow asked.

“She just does not have the enthusiasm among young people,” Caddell replied, adding:

Let’s remember, they voted in the Democratic primary. Those voting in the Democratic primary were 70-some percent, I believe, for Bernie Sanders. There has always been a problem with enthusiasm for her, for interest in the election, which is lower, and she’s just not getting those numbers.

“That’s, in fact, where Johnson and Stein have gotten a lot of support,” he noted, referring to the third-party candidates in the race, Libertarian Gary Johnson, and Jill Stein of the Green Party. “I think it’s an indicator of something.”

Caddell told Marlow the Monday morning polls were “stunning”:

The Bloomberg poll, which is conducted by the Seltzer Associates out of Iowa, who are fantastic in that state, the gold standard there, they’re doing the Bloomberg national numbers. They have it up two. As you said, you have the L.A. Times tracking poll; it’s back up to four for Trump, after coming down to a couple of points. Quinnipiac is one; it’s within the margin of error. These are margin-of-error polls. There are some really interesting numbers that are there. Our polling–which we have out–into the battleground states are matching. Ohio is +1 Trump, and Colorado’s actually several points up. Trump is up in Colorado in CNN this morning. There is a shifting going on, and part of it is in issues. … Hillary Clinton’s numbers have been extraordinarily tied, if you will, to the belief that her voters are saying the economy is good, we’re on the right track, we’re in the right direction with the country–and that is not the case. The people who are what I call “in flux,” there’s a good 20 percent of voters or more, 20 to 24 percent, depending on where you’re polling it–you have voters who believe very much against that position, who are in the float. The people who are movable right now. It’s one of the largest we have had, and I think that that is tremendously significant.

Caddell said a large number of “legitimately undecided” or “conflicted” voters would likely tune in for Monday night’s presidential debate, and they would be an audience receptive to Trump’s message of changing the status quo.

“I have said all my life, I’ve done a half-a-dozen presidential general-election debates. Debates are the vehicles of challengers,” he said. “They are the place where a challenger should be able to show that they can stand toe-to-toe with the more incumbent. And she is, relatively, the incumbent in this race, certainly the incumbent party.”

“Trump has one problem, and it has been about his temperament and his qualifications to lead. He doesn’t have to be, quote, ‘equal’ in those categories; he has to be plausible,” Caddell argued:

If he is plausible, then all of a sudden the issue range, which is basically his, kicks in–particularly the overarching one, which is the direction of the country. And as I predicted, by the way, apparently we’re seeing the voters who are concerned about this refugee, after the bombings in New York and New Jersey, and the attacks that have been going on, what is clearly becoming a distinctive and could be a voting factor is the question about refugees being allowed in. Obama wants to expand it. Hillary Clinton wants to expand it even further. Trump wants it tightened down and reduced, or stopped. The public, including many, many Democrats, stand against what Obama and Clinton want, including a number of Clinton’s own voters. And if that issue becomes prominent tonight, look for it to have a real effect on the electorate. The other point is, and I said this here last week, if Obama gets involved in the campaign, his approval ratings are coming down. And as they come down, so does Hillary Clinton. He had a choice to either stay above the fray and leave with fairly high ratings, regardless of what was going on, or become an issue himself in the campaign, to try to save her. That is what he has chosen, and therefore, as he is in the fray, his approval will come down.

Marlow asked Caddell for some closing advice about what to watch for in the debate Monday night, something “listeners might not have thought of.”

“We discussed the health thing, which is really important for her, and I don’t think anyone’s talking about it,” Caddell replied. He added:

The other is moments. I think we all look for moments in this. But the more important question for the voters is not what’s being said on Twitter. I mean, the Clinton campaign is trying to already shape they’re losing, they’re winning, whatever, I don’t know–but the fact is, they are going to be looking to see if Donald Trump is plausible, and I believe that if he does what I think he’s capable of now, then this election will move.

Caddell stressed that Trump could dominate the debate if he “keeps with the big issues, to the question of the future of the country, to the direction it’s going, and most important, to the choice: do you want to continue the path we are on?”

Breitbart News Daily airs on SiriusXM Patriot 125 weekdays from 6:00 a.m. to 9:00 a.m. Eastern.

Listen to the full audio of the interview above.