A Donald Trump presidential victory would be "unsettling" for the markets, said Larry Summers, who served in two Democratic administrations.

Summers, former Clinton administration treasury secretary, told CNBC's "Squawk Box" the nationalistic forces that led to the vote in Britain to leave the European Union make the "the possibility of a Trump victory more real" because his supporters share similar world views.

"[But] Trump has behaved in so bizarre a way [post-Brexit] it's made those risks less salient than they could have been" for financial markets, Summer said, referring to Trump's celebration at his golf course in Scotland last week for the United Kingdom vote. The real estate mogul talked about how the plummeting British pound would be good for tourism.

Summers said he does not believe Trump will become president.

The way Trump has conducted himself on the Brexit issue and past controversial remarks he's made about women and minorities makes his candidacy "look deeply disturbing," said Summers, a former economic advisor to President Barack Obama.

Summers predicts Democrat Hillary Clinton will run a better campaign than the remain camp did in Britain.