Hudson, New Hampshire (CNN) Joe Biden belittled Democratic rival Pete Buttigieg's experience as South Bend, Indiana, mayor this weekend in the harshest attack ad of the party's presidential primary to date.

Last year, though, Biden had called mayors' jobs increasingly important amid a dysfunctional Washington -- describing them as the "heartbeat" of the country and telling them that "everything lands in your lap."

"It's a reality. All the innovation in America is coming from the governors, the mayors, the county executives. It's coming up. It literally is bubbling up," the former vice president said in a January 2019 speech at the US Conference of Mayors' winter meeting in Washington.

It was one of several speeches Biden gave to the conference over the past decade. In 2014, he said he was amazed by the "tough political decisions" mayors make on a regular basis and said mayors are "the one group of elected officials that get things done, in large part because you have no option but to get things done."

Biden's ad touted his role in passing major legislation and negotiating international deals -- and mocked Buttigieg for mayoral achievements in his city of 100,000 such as new sidewalks and colored lighting underneath bridges.

On Saturday, in response to the Biden campaign's ad attacking Buttigieg, Tom Cochran, the CEO of the US Conference of Mayors, tweeted, "It's unfortunate that Vice President Biden is belittling the role" of mayors.

Buttigieg responded to the ad at a Democratic gathering in Manchester on Saturday night, telling the crowd Americans in small towns "are tired of being reduced to a punchline by Washington politicians and ready for somebody to take their voice to the American capital."

Buttigieg has often acknowledged how unusual it is for a small-city mayor to mount a top-tier presidential campaign.

Two years ago in Illinois, when he filled in for an ill Biden at a Democratic gathering months before the two would become presidential rivals, Buttigieg compared his then-modest stature as "a 36-year-old small-city midwestern mayor with an unpronounceable name" to "an American giant like Joe Biden."

Both the Biden and Buttigieg campaigns declined to comment for this story.

Biden, in his 2019 speech to the US Conference of Mayors , joked about quickly leaving the only local elected office he held.

"I was a local official," Biden said, referring to a short tenure on the New Castle County Council in Delaware in the early 1970s.

"I ran for United States Senate because being a local official was too hard," he said. "You all think I'm kidding -- I'm not kidding. They know where you all live."

He said that "the cities of America are the heartbeat of America. They are the economy of America."

"Everything about what kind of nation we are can be determined by looking at our cities," Biden said. "The rest of the nation is really important to state the obvious -- rural areas, suburban areas -- but the heartbeat, a heartbeat of the country are the mayors, and everything lands on your lap."

"You affect the quality of life and you're viewed as the person most responsible for the quality of life in your city, more than anyone else in politics more than anywhere else in government," Biden said in 2019.

"You know, picking up the garbage and make sure it's picked up -- it's not Democrat or Republican. It's not a matter of whether or not you -- if the streets aren't safe, it doesn't matter what your politics are. And if you don't have good schools, people are in trouble and you've, you've been, the whole world is sort of dropped on you."

In 2014 , telling the group that mayors face "unique challenges when it comes to building infrastructure and creating jobs," Biden pointed out that then-President Barack Obama had nominated a big-city mayor, Charlotte's Anthony Foxx, to be his transportation secretary.

"And also, I've gotten a chance to work directly with so many of you during the Recovery Act," Biden said. "The only reason it worked, the only reason there was less than 1% waste or fraud -- including with our Republican friends who investigated -- is because of you. You made it work. You're used to getting things done on time -- mostly under budget -- and getting answers back to people immediately."

"And it never ceases to amaze me the tough political decisions, you guys and women, you make every single day in doing your job -- to save your neighborhoods, to rebuild and balance your budgets, and to bring jobs back to your communities," he said.

And in 2010 , as Biden was leading the effort to disburse stimulus funds and rebuild the economy, he praised mayors' role in the economic recovery.

"I must tell you, I've been impressed by the competence, the management skills that so many of you have demonstrated in incredibly difficult financial times and difficult circumstances," he said. "The leadership of each and every one of you in this room is the basis upon which I think we're going -- this recovery is going to grow."