South Bend, Ind., Mayor Pete Buttigieg responded Sunday after Rep. Alexandria Ocasio-Cortez, D-N.Y., took issue with the 2020 Democratic presidential hopeful's approach to fundraising.

The far-left congresswoman accused Buttigieg of protecting a system of "big money politics" instead of trying to make it a thing of the past, touting the merits of "small-dollar grassroots campaigns," which she said were more successful than his. Buttigieg said he's quite familiar with grassroots campaigning, but said it will take more than that for Democrats to succeed in 2020.

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“Well, first of all, you don’t go from mayor of South Bend to a competitive presidential candidate without knowing a thing or two about grassroots campaigning," Buttigieg told "Fox News Sunday" in response to Ocasio-Cortez's tweet, posted last week. "My campaign is fueled by the contributions of almost 600,000 individual donors and most of those are small contributions."

Ocasio-Cortez posted her remark while retweeting a quote from Buttigeg that criticized Sen. Elizabeth Warren's plan to stay away from large donations. Buttigieg had said, "We're not going to beat Trump with pocket change." Ocasio-Cortez described that comment as insulting to small donors, but Buttigieg explained his stance on Sunday.

"What I’m saying is we can’t go into this fight against Donald Trump with one hand tied behind our back," he said, pointing out Trump and his supporters have raised $125 million. "They will pull out all of the stops to stay in power, and I think we have a responsibility to the country to make sure that we go into this fight as Democrats with everything that we've got and not unilaterally disarm it."

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Buttigieg recognized that campaign finance reform is needed, but claimed Democrats have to gain power first in order to make it happen.

Earlier in the conversation, Buttigieg took a shot at far-left opponents like Sen. Bernie Sanders, I-Vt., who just received Ocasio-Cortez's endorsement. Buttigieg warned that radical approaches to issues like health care will alienate voters and make it more difficult to win.

"I think that we have a chance to build an American majority around bold action, but it is the case that we could wreck that majority through purity tests," Buttigieg said. "Take the example of this Medicare question. I’m proposing Medicare for all who want it. It means we create a version of Medicare, everybody can get access to it, and if you want to keep your private plan we’re okay with that. I think that’s a better policy than kicking people off of their plan, but I also think that it’s something that more Americans can get behind.”

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Lamenting the divided state of the political climate in the U.S., Buttigieg said that going too far to the left can make matters worse.

"We can govern in a very bold and forward-leaning direction, but we’ve got to make sure we do it in a way that moves toward unifying rather than further polarizing the American people.”