It was August 2017, when he was campaigning for a second term in office, when Mayor Bill de Blasio made a promise to voters during his Democratic primary:

De Blasio: I want to serve for four more years.

Moderator Grace Rauh: Will you pledge to serve for four more years?

De Blasio: I will serve for four full years.

Now, almost a year and a half later, de Blasio is not ruling out a 2020 presidential run — and he says a changing world is why he may change his tune.

"Not ruling it out means not ruling it out," the mayor said when pressed for more details during an unrelated news conference Monday. "My current plan is to do this job with great intensity all the way until the end of 2021. But we're in an ever-changing environment, and I was trying to be honest: I'm simply not ruling it out."

The comments came after de Blasio said during an appearance on CNN's State of the Union on Sunday that he wouldn't rule out a 2020 run for the White House.

"I never rule things out because you never know what life brings," he said in the interview.

"The world changed," the mayor said when asked what changed since his 2017 promise. "Today, when I say my plan is to serve until the end of 2021, I mean it. That is my plan. But I also see an extraordinary environment, where we're seeing challenges we never saw before. We're seeing dangers we never saw before. You know, it's a time when it makes sense to rethink what's going on and how to address it."

The mayor didn't offer a timeline for a potential decision, saying this was just beginning of the process.

The Democratic field for president is slowly starting to take shape ahead of the first primary debate in June, and anyone seriously considering a campaign would likely need to announce their run within the next few months.

If de Blasio were to run, he would join what is expected to be a crowded primary field, which may include New York Sen. Kirsten Gillibrand and a former New York City mayor.

When reporters asked if he was qualified to be president, the mayor said it was mayors across the nation who were setting policy and moving progressive agenda forward.

De Blasio has injected himself into the national political conversation over the past week, after he unveiled plans earlier in the week to expand health coverage to low-income New Yorkers and mandate paid vacation days for private-sector workers. The health care plan is an effort to expand and rebrand the public health care services that the city already provides, and the City Council would have to approve the paid time off mandate.

De Blasio has been pitching his progressive agenda to national media outlets and almost egging on any speculation he may be considering a run for the Oval Office.

In fact, he is planning to take his agenda on the road yet again, although we still do not have details of his renewed travel schedule or if Iowa is on the itinerary.

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