Robert F. Kennedy Jr., the nation’s most prominent vaccine skeptic, courted the votes of senators considering whether to confirm him as health secretary by promising, repeatedly and in writing, to do nothing “that makes it difficult or discourages people from taking vaccines.”

Seven months have passed since he took office. In that time, Mr. Kennedy has delivered a lukewarm endorsement of the measles vaccine; dismantled a panel of experts who make vaccine recommendations to the government; taken steps that will effectively restrict access to Covid-19 vaccines; canceled $500 million of grants and contracts for the development of mRNA vaccines; and, just last week, forced out the new director of the Centers for Disease Control and Prevention because she disagreed with him on vaccine policy.

On Thursday, Mr. Kennedy will come before the Senate again with his department in turmoil.

President Trump memorably said he would let Mr. Kennedy “go wild on health.” Now the health secretary is expected to face questions from lawmakers who suggest he may have gone a little too wild. Uneasy Republicans, who until now have been reluctant to criticize Mr. Kennedy, are wondering aloud whether his actions contradict the pledges of restraint he made to win the job. Angry Democrats say his tenure is a danger to public health and have called on him to resign.

“He’s got to reconcile what he said during his confirmation process with what we’ve seen over the past few months, particularly on vaccine policy,” said Senator Thom Tillis, Republican of North Carolina and a member of the Senate Finance Committee, which is convening the hearing.

Senator Ron Wyden, the top Democrat on the finance panel, asked the questions that elicited Mr. Kennedy’s written pledge to not restrict vaccine access. Mr. Wyden said in an interview on Wednesday that he agreed with others in his party who have called on Mr. Kennedy to resign, and he was unequivocal when asked whether Mr. Kennedy had lived up to his word months ago.