"I'm surprised how many people think Hillary's going to win and that she's a shoo-in. She has a lot of flaws," Valliere said. "Sanders is still not going away. What if the economy continues to barely grow?"
A weaker economy is one of the biggest threats to Clinton. The other would be if there were another major terror attack, whether domestic or foreign. "His (Trump) numbers always go up when you see something like that," said Valliere.
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Other pundits agree that those are two of the biggest factors that could swing the presidency toward Trump.
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Valliere said another issue also could turn voters against Clinton, who is closely tied to the Obama administration: Significant increases in health-care premiums expected in 2017 will be announced in the fall during the thick of the race.
Trump's health-care plans are "incoherent," but for now all he has to do is "blast Obamacare," Valliere said.
Valliere said Clinton was still the favorite as she had the Electoral College advantage.
But Trump's victories have already exceeded expectations, and he is closing the polling gap with Clinton. In one new poll by Rasmussen, he even pulled ahead of her with 41 percent support to her 39 percent.
Valliere said Trump would be "going for the jugular," attacking Clinton personally as well as her judgment about her email server. She already has high negatives — at about 55 percent, though Trump's are closer to 65 percent.
Candidates typically move closer to the center as the election gets closer, but Valliere noted that Clinton had already moved more to the left to compete with Sanders.
Correction: This story was revised to correct when Cruz suspended his campaign. It was Tuesday night.