Story highlights Republicans have won all four of the special elections held under President Trump

Trump is "doing better with the voters than Democrats think he is," David Gergen says

(CNN) As President Donald Trump celebrated Republican victories in two special congressional elections on Wednesday, one man who advised presidents from both parties warned the results "should be a wake-up call for Democrats" that Trump could be on his way to re-election in 2020.

Republican Ralph Norman defeated Archie Parnell in Tuesday's special election for South Carolina's 5th congressional district, and in Georgia, Republican Karen Handel defeated Democrat Jon Ossoff in the most expensive House race in history. Republicans have won all four of the four special elections that have been held under President Trump.

During an appearance on CNN's "Erin Burnett Outfront," David Gergen, who advised former Presidents Richard Nixon, Gerald Ford, Ronald Reagan and Bill Clinton, admitted that off-year elections like this may not be the best predictor of what could happen in the 2018 midterms. But he said it should send a strong signal to Democrats thinking about taking back the White House.

"What's really important is that Donald Trump has seized the narrative back, that he's doing better with the voters than Democrats think he is," he said. "It should be a wake-up call for Democrats. It is possible that he could actually get re-elected if Democrats aren't careful."

Gergen has criticized Trump on a variety of issues, including his withdrawal from the Paris climate agreement and his firing of James Comey as FBI director, a fact he acknowledged during his appearance. But he said there is no doubt that "this is one occasion when we ought to say he deserves to take a victory lap."

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