Former second lady Jill Biden lashed out at President Donald Trump on Saturday, accusing him of "bullying" and being "afraid" to run against her husband.

The wife of former Vice President Joe Biden told MSNBC's "Up with David Gura" that Trump was wrong to attack 16-year-old environmental activist Greta Thunberg, who was named Time magazine's person of the year before she was immediately mocked by the president.

"That's bullying," Jill Biden told MSNBC. "Look at what the president did this week with that 16-year-old girl, Greta. You can't attack children. That's the bottom line."

The Bidens have been center stage of impeachment proceedings against Trump, who asked the Volodymyr Zelenskiy to investigate the former vice president and his son Hunter and a debunked theory about the 2016 election. The administration placed a hold on military funding to Ukraine that had been approved by Congress at roughly the same time.

Hunter Biden served on the board of a Ukrainian energy company from 2014 to 2019 and Trump has asserted, with no evidence, that the former vice president used his office to advance his son’s business interests.

"We knew it was going to be tough. Our family knew it was going to be tough, but we could never have imagined that it would turn into Donald Trump ... asking a foreign government to get involved in our elections," Jill Biden said Saturday. "And I think it just proves he's afraid to run against my husband, Joe Biden."

Joe Biden, who recently turned 77, would be 78 and about three months if he were elected and took office in January 2021. Jill Biden dismissed reports that her husband would consider serving just one term.

"He has a lot of energy," she said. "Most of the time I have to say to him in the morning like, 'Joe, just wait until I have my coffee until you start with this idea or that idea.'"

President Trump is now 73.

With the Iowa caucuses about seven weeks away, Biden is locked in a close battle with South Bend, Indiana, Mayor Pete Buttigieg, Sen. Elizabeth Warren, D-Mass., and Sen. Bernie Sanders, I-Vt.