Mayor Bill de Blasio stood firmly Tuesday behind a weekend pronouncement that he isn't ready to endorse former boss Hillary Rodham Clinton's presidential bid and wants to first see her vision for curbing income inequality.

"You might have a situation where you consider someone a friend and you have deep respect for them, but you have a substantive issue you have to resolve," he said at an unrelated news conference in the Bronx. The widening gap between rich and poor is a "dangerous situation" that threatens the "fate of the United States of America," de Blasio said.

The mayor said he had notified Clinton and her team of his position before appearing Sunday on NBC's "Meet the Press." He said of the former secretary of state's yet-to-be-unveiled agenda: "We need to see the substance.

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"I don't think there's any surprise" in his hesitation to give his nod, he said.

The mayor, a standard-bearer of the Democratic Party's progressive wing, was the campaign manager behind Clinton's successful 2000 campaign for U.S. senator. His family and the Clintons have had a yearslong friendship. President Bill Clinton presided over de Blasio's mayoral inauguration in January 2014.

Other Democrats have criticized de Blasio for failing to immediately throw his weight behind Clinton. Rep. Sean Patrick Maloney, who represents New York's Hudson Valley, said in an interview on Albany-based WAMC radio Tuesday that de Blasio needs "his head examined."

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De Blasio insisted he is "a loyal person in life" and "in politics."

But he doubled down on his call for Clinton and others in leadership positions to champion causes such as more progressive taxation, raising wages and benefits and a "willingness to tax the wealthy" to fund infrastructure and education.

His remarks came as he prepares to travel to the Midwest for three speeches on closing the wage gap. He will speak at a public service lecture series for the University of Nebraska Foundation in Omaha; and at Drake University and at a Progress Iowa gathering in Des Moines.

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De Blasio said that he intends to use the "bully pulpit" to advance his progressive agenda in Iowa, an early-voting state, and elsewhere in the nation, complaining Washington has ignored the problems.

"Someone find me something that's come out of Washington, D.C., in recent years to address income inequality," he said. "Good luck with that."

The country was just entering the Great Recession when Clinton last ran for president eight years ago and now faces deeper economic inequalities, de Blasio said. He said Clinton's stances on the issues must be updated to reflect that new reality.

"It's normal to want to hear more," he said.