The political organization founded by Mayor Bill de Blasio canceled its presidential forum planned for next month in early-voting Iowa, a political embarrassment for a mayor with ambitions of influencing national dialogue.

The Progressive Agenda Committee, which de Blasio founded in the spring, had scheduled a bipartisan forum on income inequality at the University of Iowa on Dec. 6. But no candidate had publicly committed to attending the forum and de Blasio was criticized for seemingly prioritizing national interests ahead of his municipal responsibilities.

On Tuesday, the committee pulled the plug on the event.

Its executive director portrayed the cancellation as a victory, saying the forum was no longer needed because income inequality had already become a major part of the 2016 conversation.

"For the past several months, addressing inequality has become more and more a central part of the Democratic presidential contest, most recently on display during Friday's forum in South Carolina," Geri Prado said in a statement. "Some of these issues have even made their way into the GOP contest."

Prado added that the committee -- which includes left-leaning labor leaders, activists and politicians such as U.S. Rep. Raul Grijalva of Arizona and Minnesota Gov. Mark Dayton -- would soon establish a leadership council and turn toward policy and elections on local, state and federal levels.

But the cancellation appeared to be the latest misstep for de Blasio, a Democrat, in his efforts to impact the 2016 race.

He first drew criticism when he did not immediately endorse former Secretary of State Hillary Rodham Clinton, who had plucked him out of relative obscurity to manage her 2000 U.S. Senate campaign. He had maintained close ties to the Clintons -- the former president administered the oath of office during the mayor's January 2014 inauguration -- and his refusal to back his former boss was viewed by many as disloyal.

De Blasio argued that he wanted to highlight liberal causes like paid sick leave, universal prekindergarten and reining in the national debt and suggested that he hoped Clinton would move to the left on many issues. Even as she did -- likely in response to the unexpected liberal challenge from U.S. Sen. Bernie Sanders of Vermont -- that did not sway de Blasio to back her, even as members of his own City Hall staff urged him to do so.

In the meantime, The Progressive Agenda scheduled the forum and announced plans to invite the top five candidates in each party based on Nov. 1 polls. But as de Blasio's own poll numbers fell, none of the presidential candidates signaled their attention to attend.

Citing her opposition to the Trans-Pacific Partnership trade agreement, de Blasio finally endorsed Clinton last month, doing so in an early Friday morning TV interview during a week in which the nation's largest city was preoccupied by the killing of a police officer, the anniversary of Superstorm Sandy and the Mets' World Series run. His endorsement and vow to vigorously campaign for Clinton appeared to remove any final incentive for the other candidates to attend the forum.

A spokeswoman for de Blasio's administration declined to comment about the cancellation.