You don’t need a Ph.D. to be in Congress. But some advocates for the sciences believe not enough lawmakers have them.

The retirement of Rep. Rush Holt (D., N.J.), who for 16 years was the House’s resident astrophysicist, represents the latest in a string of departures by members trained in the sciences.

His exit leaves Reps. Bill Foster (D., Ill.) and Jerry McNerney (D., Calif.) as the only remaining members who hold doctorates in the natural and hard sciences out of the 535 senators and representatives in the 114th Congress, according to the Congressional Research Service.

“I’m going to have to carry the torch on that myself,” said Mr. Foster, a specialist in high-energy particle physics, about representing the interests of the scientific community in matters such as funding.

While physicians are relatively more numerous, with 19 medical doctors in the new Congress, attracting natural scientists to shift into national politics is unusual.