President Bill Clinton's administration appointed Bill de Blasio to the U.S. Department of Housing and Urban Development. Hillary Clinton elevated de Blasio to be campaign manager of her U.S. Senate bid. And both Clintons appeared at de Blasio's inauguration as New York City mayor and have supported his agenda, both publicly and privately.

With those close ties as the backdrop, the mayor made an appearance Sunday on "Meet the Press" in the hours before Hillary Clinton announced her candidacy for president, and he raised eyebrows across the political world by declining to endorse his former boss.

De Blasio, a Democrat more liberal than Clinton on most issues, said he would withhold his blessing "until I see an actual vision of where they want to go."

"She's a tremendous public servant," the mayor continued. "I think she is one of the most qualified people to ever run for this office. And by the way, thoroughly vetted, we can say that. But we need to see the substance."

The backlash was immediate.

Hilary Rosen, a Clinton ally, wrote on Twitter: "@BilldeBlasio's self aggrandizing on #MeetthePress at @hillaryclintons expense won't go unnoticed. #Ridiculous."

Former congressman Anthony Weiner, whose wife is top adviser to Clinton, told The Wall Street Journal that de Blasio should treat the Clintons like family and "you don't ask a family member to lay out her resume before you decide to support her."

"She (was) working on a progressive vision of health care when Bill de Blasio was still smoking pot at NYU or wherever he went," said Weiner, who ran against de Blasio for mayor in 2013.

And as other New York politicians — including Gov. Cuomo and its U.S. senators — quickly endorsed Clinton, both city tabloids ripped the mayor on their front pages, going with the headlines "Hil's Fury" and "Traitor" (New York Post) and "Stabbed in de Back" (New York Daily News).

Much of the criticism suggested that de Blasio — who has been widely expected to be an active campaigner for the former secretary of state — was being ungrateful to the Clintons and used the "Meet the Press" appearance to bolster his own national profile at the expense of his former boss. Others noted that he backed Cuomo instead of Cuomo's far more liberal opponent Zephyr Teachout in an apparent attempt to keep the peace with the state's most powerful Democrat.

"I think it's really problematic that it came off like it was all about him," said Jeanne Zaino, political science professor at Iona College. "His own political future — not hers, not the good of the party — seemed to matter most."

De Blasio has made no secret of his efforts to become, along with U.S. Sen. Elizabeth Warren of Massachusetts, a leading voice for the ultraliberal wing of the Democratic Party. He has taken steps to raise his profile, including making a bid for the 2016 Democratic National Convention, hosting a meeting of liberals at Gracie Mansion this month and traveling to Nebraska and Iowa — site of the first nominating caucus — later this week to champion the fight against income inequality.

Clinton will also be in Iowa this week as she begins her campaign, but the two are not expected to meet.

But some political observers believe that de Blasio's reluctance to immediately offer his endorsement was calculated and clever, increasing the value of it in the months ahead and potentially getting Clinton to move to the left on some issues. It also, according to veteran political strategist Hank Sheinkopf, might actually increase the potency of his support down the road.

"De Blasio is setting himself up as the progressive seal of approval and that means that any candidate he blesses checks all the right liberal boxes," Sheinkopf said. "So, when he's eventually out there campaigning for her — which of course he will be — he acts as a barrier to any attacks from the left."