De Blasio said Clinton is "more prepared arguably than anyone who has ever run for president," but he added that his choice depends on what the candidates say about their grand plans.
"I've been very impressed by what Hillary Clinton has put out," he told CNBC's "Squawk Box." "With each passing week, she has added to her vision in a very compelling manner."
But, he added, "there are still a few areas where I think we have to fill in the blanks and get a better sense of where things are going."
De Blasio served as campaign manager for Clinton during her successful run for the U.S. Senate in 2000. Three years earlier, then President Bill Clinton appointed de Blasio as regional director for the U.S. Department of Housing and Urban Development for New York and New Jersey.
The mayor said he was satisfied that the candidates the Democrats have are the right ones, adding that they are speaking to the core economic realities that Americans are facing.