South Bend, Ind., Mayor Pete Buttigieg Pete ButtigiegBillionaire who donated to Trump in 2016 donates to Biden The Hill's Morning Report - Sponsored by Facebook - GOP closes ranks to fill SCOTUS vacancy by November Buttigieg stands in as Pence for Harris's debate practice MORE on Monday said he was "nervous" about how the U.S. would appear before the United Nations ahead of President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE's appearance at the General Assembly meeting in New York City.

Buttigieg compared Trump to a child during his remarks, and suggested the U.S. would not provide leadership at the U.N. under the Republican president.

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"Having seen what happened to us at the G-7 where the American president was basically patted on the head and told to go enjoy himself while the grown-ups sat down to talk about climate change, I am very nervous about how the U.S. is going to look at the U.N. General Assembly that is going to convene right here in the United States," Buttigieg said in an address to supporters in Iowa Falls.

"Will it be one more time where the seat for U.S. leadership is empty? Or are we going to demonstrate that we're a country that keeps its word and is prepared to lead?" he continued. "At the end of the day, you can either resent the rest of the world or you can lead the rest of the world, but you can't do both."

While Trump was notably absent from a discussion on climate change and biodiversity at the G-7 meeting in France last month, he did make an impromptu appearance at a U.N. summit on climate change on Monday.

The appearance comes as climate protesters stormed the streets of Washington, shutting down streets across the nation's capital.

The president's appearance at the U.N. meeting this week is also being closely watched amid the growing controversy over his past conversations with Ukrainian leaders over an investigation into former Vice President Joe Biden Joe BidenFormer Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick Bloomberg rolls out M ad buy to boost Biden in Florida MORE and his son, Hunter Biden.

Trump is also likely to address growing tensions with Iran, in addition to trade relations with China.