Dr. Dave Weldon, a former Republican representative and President Trump’s pick to lead the Centers for Disease Control and Prevention, will appear before the Senate health committee on Thursday, the first time an agency director has been subject to the confirmation process.

Dr. Weldon, 71, is perhaps the least known of the men nominated to lead major agencies at the Department of Health and Human Services. But he is the one aligned most closely with Robert F. Kennedy Jr., the nation’s new health secretary.

Dr. Weldon, like Mr. Kennedy, has long questioned the safety of certain vaccines, and the two have maintained a 25-year relationship. The health secretary has cited Dr. Weldon’s criticisms of the C.D.C. along with his own.

Dr. Weldon served in Congress for 14 years, from 1995 to 2009. His signature legislative accomplishment was the Weldon Amendment, which bars health agencies from discriminating against hospitals or health insurance plans that choose not to provide or pay for abortions.

He also argued that abstinence is the most effective way to curb sexually transmitted infections. Cases have soared in recent years and only began to level out in 2023.

Dr. Weldon’s hearing takes place amid significant measles outbreaks in Texas and New Mexico, which have infected more than 250 people and claimed two lives; a flu season that led to record numbers of hospitalizations; and the potential for a bird flu epidemic.