Democrat presidential candidate Julián Castro argued Thursday that President Donald Trump mocking teenage climate change activist Greta Thunberg’s TIME magazine “Person of the Year” cover makes it clear he is “unfit for office.”

“The president of the United States is mocking a 16-year-old on Twitter,” Castro, who previously served as Secretary of Housing and Urban Development (HUD), wrote on Twitter in response to the president’s tweet. “It’s clear why one of them is the Person of the Year and the other is unfit for office.”

The president of the United States is mocking a 16-year-old on Twitter. It’s clear why one of them is the Person of the Year and the other is unfit for office. https://t.co/1DbeTUtwii — Julián Castro (@JulianCastro) December 12, 2019

Earlier Thursday, President Trump called the magazine cover “So ridiculous,” and joked of the Swedish 16-year-old: “Greta must work on her Anger Management problem, then go to a good old fashioned movie with a friend! Chill Greta, Chill!”

So ridiculous. Greta must work on her Anger Management problem, then go to a good old fashioned movie with a friend! Chill Greta, Chill! https://t.co/M8ZtS8okzE — Donald J. Trump (@realDonaldTrump) December 12, 2019

Thunberg responded by changing her Twitter biography to read: “A teenager working on her anger management problem. Currently chilling and watching a good old fashioned movie with a friend.”

Donald Trump Jr., the president’s eldest son, also criticized Thunberg’s selection for the annual accolade, stating the pro-democracy protests in Hong Kong were more deserving.

“Time leaves out the Hong Kong protesters fighting for their lives and freedoms to push a teen being used as a marketing gimmick,” Trump Jr. tweeted. “How dare you?”

Time leaves out the Hong Kong Protesters fighting for their lives and freedoms to push a teen being used as a marketing gimmick. How dare you? https://t.co/dnV9mESM6G — Donald Trump Jr. (@DonaldJTrumpJr) December 11, 2019

President Trump previously mocked Thunberg in September, joking she seems like “a very happy young girl looking forward to a bright and wonderful future” after she delivered a fiery speech at the United Nations climate summit in New York City in which she accused world leaders of abdicating on their responsibility to combat global warming.