Mr. de Blasio, who is running for re-election next year, is not alone among mayors who see a coalition of cities as a potentially powerful bulwark against drastic changes in Washington.

In conference calls and informal meetings, mayors from Seattle to New Orleans have been discussing how to best position their cities as a kind of bloc of island nations, with shared concerns over the prospect of diminished federal support for urban centers, and of major shifts in policy on immigration, public safety and climate change.

“We’re taking a no-excuses strategy, as opposed to lamenting President-elect Trump’s win and his policy positions,” said Mayor Kasim Reed of Atlanta, a Democrat. “We think that by joining together that we can have a very strong countermessage.”

But few identified Mr. de Blasio — or any single big city politician for that matter — as the best person to lead the charge.

“The power of mayors right now is that they’re seen as the last functioning form of American government,” said R. T. Rybak, a vice chairman of the Democratic National Committee and onetime mayor of Minneapolis. “If it is seen that mayors are following the dictates of someone in New York — it’s deeply appreciated, but it doesn’t have the same power as if it’s springing from Main Street.”

The limits of Mr. de Blasio’s influence over congressional Republicans could already be seen in his as-yet-unsuccessful attempts to get substantial federal reimbursement for the cost of added security in and around Trump Tower on Fifth Avenue, where Mr. Trump has his home and offices and where much of the presidential transition work is taking place. Mr. de Blasio sought $35 million for the costs through Inauguration Day; Congress so far has allocated about one-fifth of that amount.