An aide said he will file the required paperwork on Thursday.
"It was wrong 100 years ago and it’s wrong today,” he writes.
Forty-six percent of Americans identify as Democrats or leaning Democratic, Gallup found.
De Blasio told the AP that Trump's comments were "disgusting and offensive."
The 2016 Republican presidential contender refused to back down in a Wednesday interview.
The Democratic presidential candidate reportedly spoke to 10,000 supporters Wednesday.
“I think the American people are really going to be pushing back on this."
“He's not hurting other candidates. He is risking the Republican brand,” says one strategist.
Jesse Byrnes 07/01/15 06:05 PM EDT
"It's just not in the cards," the veteran lawmaker said.
The amount of money being spent is "unreal," he said.