Senator Bill Cassidy, the chairman of the health committee, said Americans should not trust a possible change to the childhood vaccine guidance.

At a Senate health committee hearing, Dr. Susan Monarez painted a picture of Health Secretary Robert F. Kennedy Jr. as a man wedded to his own ideology and uninterested in government scientists.

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Senator Bill Cassidy, a Republican physician and vaccine proponent who is facing a primary challenge from the right, has a fraught relationship with the health secretary.

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The killing of the right-wing activist deepened lawmakers’ fears of violence directed against them, and the toxic discourse that has fed such threats.

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Some Republicans cheered the move as a justified step by Israel, while others said they were worried the attack could strain relations with a close U.S. ally.

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At a contentious hearing with Robert F. Kennedy Jr., the health secretary, Republicans who voted to confirm him were among his critics.

A three-hour hearing before the Senate Finance Committee revealed that the health secretary was on uncertain ground even with some Republicans who voted to confirm him.

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