New York City Mayor Bill de Blasio said Sunday he is warming steadily to former Secretary of State Hillary Rodham Clinton's vision as a presidential candidate, but he still hasn't heard enough to offer his endorsement.

"I think she's running a very good campaign because her vision is getting stronger and stronger," de Blasio said on ABC's "This Week." The Democrat said he is impressed with her ideas as they are unveiled week by week.

De Blasio added, however, that there are a few more areas that Clinton, whose 2000 U.S. Senate campaign he managed, needed to "flesh out," notably wages and benefits and trade.

PhotosFunny and revealing quotes from the Aug. 6 GOP debatesQuizQuiz: How well do you know the 2016 contenders?More coverageThe 2016 campaign: Complete coverage

"This Week" host George Stephanopoulos suggested that the progressive mayor was caught between his head and his heart, or loyalty to his former boss and political affinity for her opponent in the Democratic primary, Vermont independent Sen. Bernie Sanders.

De Blasio said he will make his decision on who to endorse in 2016 just as anyone else would: "Look at the vision, look at the facts, look at capacity."

De Blasio also served in President Bill Clinton's U.S. Department of Housing and Urban Development.

advertisement | advertise on newsday

He again urged Vice President Joe Biden, who is mulling a late bid for president, not enter the race for the White House. "The Democrats are in a strong, solid place right now" with their field of candidates, de Blasio said.

The mayor first declined to endorse Clinton on a Sunday morning talk show in April, when he appeared on NBC's "Meet the Press" the morning she made official her bid and said he needs to see "the substance." The interview was booked long before it was known that the former first lady would announced her campaign.

In recent months, de Blasio has said he approves of the speeches she has made on criminal justice and immigration.

@Newsday

Many other New York elected officials, from Gov. Andrew M. Cuomo to Sen. Charlie Schumer to Public Advocate Letitia James and City Council Speaker Melissa Mark-Viverito, have voiced their support for Clinton's candidacy.