Mayor de Blasio — not Hillary Rodham Clinton — will be the Democratic nominee for president in 2016.

That’s the jaw-dropping prediction being made by New York’s top Republican, state GOP Chairman Ed Cox, who, as son-in-law to former President Richard Nixon, knows a thing or two about national politics.

Cox, citing information provided by a prominent “Democratic lobbyist,’’ told friends and associates in recent days that freshman Mayor de Blasio’s effort to promote himself as the leader of the “urban progressive centers of the nation’’ is part of a well-oiled plan to prepare for a presidential run.

“It’s like Barack Obama; he was a brand-new freshman senator, and he ran for president and won. I think de Blasio is going to do it,’’ Cox said at a recent gathering, a source told The Post.

Cox also cited de Blasio’s remarkably close but dicey ties to the controversial, and racially divisive, Rev. Al Sharpton, an Obama friend and one-time Democratic presidential hopeful who has a vast national political network, as evidence that the mayor sees his political future as somewhere beyond New York City, the source said.

“Cox has been pointing out that Sharpton is back and forth to the White House and serves as an emissary for de Blasio,’’ said the source, who pointed out that former top Sharpton aide Rachel Noerdlinger continues to serve as chief of staff to de Blasio’s wife, even as Noerdlinger has become a political liability.

Cox also found backing for his prediction last week in a provocative Huffington Post column in which the mayor — sounding like a national spokesman for left-of-center “progressive’’ Democrats — blamed the sweeping Republican election victories earlier this month on the failure of Democrats to be progressive enough.

“This year, too many Democratic candidates lost sight of those core principles — opting instead to clip their progressive wings in deference to a conventional wisdom that says bold ideas aren’t politically practical,’’ de Blasio wrote.

Cox told associates that the “small blue dots’’ on the national electoral map containing the nation’s biggest cities will dominate the 2016 Democratic primary and that de Blasio, “the leader of the urban Democratic Party who holds the second-most important job in America,’’ would strongly appeal to voters there.

As for Clinton, Cox, a Manhattan lawyer whose life has been steeped in politics ever since he married then-President Nixon’s daughter in 1971, contends the former secretary of state is out of step with party progressives and doesn’t have the political skills of her husband.

“The national Democratic Party is going hard left. It’s Obama’s party, and that’s why [freshman Mass. Sen.] Elizabeth Warren gets them excited,’’ Cox said recently. “But Hillary voted for the Iraq war and then doubled-down by saying we should have gotten more involved in Syria and talked about businesses not creating jobs. She’s trying to ride in as a moderate when the party’s gone hard left,’’ he continued.

Cox wouldn’t directly respond to questions about his remarks, but he told The Post, “De Blasio is a man with huge national ambitions, and for those ambitions, New York City is just a stepping stone.

Meanwhile, some Democrats are yelling “hypocrite’’ at de Blasio over his Huffington Post essay.

The Democratic critics charge that de Blasio didn’t exactly stick to his “core principles’’ with his own endorsement of Gov. Cuomo, who has been widely criticized by party “progressives’’ for failing to support state Senate Democrats.

“People thought it was incredibly hypocritical for the mayor to go around lecturing progressive Democrats across the country in the Huffington Post about how they need to have a backbone, when he bent over backwards to help people like Cuomo,’’ said a prominent Democratic activist with ties to de Blasio.