A number of Senate Democrats have indicated that they might oppose President Barack Obama’s choice of Vivek Murthy for the post of U.S. Surgeon General, according to Senate aides, putting the nomination at risk over the issue of gun control.

Dr. Murthy’s nomination is opposed by the National Rifle Association, the country’s largest gun lobby, because he has expressed support for gun control, calling it a public-health issue.

Chris Cox, executive director of the group’s political arm, said in a Feb. 26 letter to Senate Majority Leader Harry Reid (D., Nev.) and Senate Minority Leader Mitch McConnell (R., Ky.) that the NRA opposed Mr. Murthy because of his support for a “wide array of gun control policies,” including a federal ban of certain semiautomatic firearms, often called assault weapons, and their ammunition.

“Given Dr. Murthy’s lengthy history of hostility towards the right to keep and bear arms, along with his calls for the full weight of the federal government’s health apparatus to be used to target lawful gun ownership, there is little reason to believe that he would not work to further a gun control agenda if confirmed as Surgeon General,” Mr. Cox wrote.