In a tweet later Thursday, Mr. Yang, who peppers his stump speech with jokes and has played basketball and crowd-surfed on the campaign trail, thanked Ms. Williamson and praised her approach. She is expected to appear with him and speak at a town hall event in Fairfield, Iowa, on Friday, his campaign said.

“I have learned a lot from Marianne and continue to do so,” tweeted Mr. Yang, who became friends with Ms. Williamson during their months on the campaign trail, sometimes exchanging phone calls and texts. “She answers questions that many of us haven’t even thought to ask. Very grateful for her friendship and support in this important time.”

In a statement provided to The New York Times on Thursday afternoon, Ms. Williamson emphasized she was “not endorsing any presidential candidate at this time,” adding that she supports “all the progressive candidates.”

“I’m appearing with Andrew Yang in Fairfield because I know the institutional obstructions to his candidacy and I want to see him continue in the race past Iowa,” she said in the statement.

After a modest but steady rise in the polls, Mr. Yang has struggled to get beyond the mid-single digits in both national surveys and polls of the critical early voting states. Those struggles kept him off the debate stage this month in Iowa, a first for his campaign. He remains two qualifying polls short of making the cut for February’s Democratic debate in New Hampshire, a state that he has said he believes he can do particularly well in and from which he will need to gain momentum to fuel his campaign into later stages of the race.