MIAMI — The nomination of a gay black Miami judge to the federal bench will not move forward after Senator Marco Rubio announced he was withdrawing his support over concerns about the judge’s actions in two criminal cases.

Without Mr. Rubio’s approval, Judge William Thomas’s nomination to the Federal District Court for the Southern District of Florida, in Miami, is effectively blocked. Judge Thomas, who serves on the Miami-Dade Circuit, was nominated by President Obama, with Mr. Rubio’s backing, more than 10 months ago.

Senator Bill Nelson, Democrat of Florida, signed off on the nomination on July 24 after a background check raised no red flags. For a confirmation to proceed, nominees must secure the approval of both United States senators in their home state.

Supporters of Judge Thomas, who grew up on welfare in Pennsylvania in a family of 10 children, said Mr. Rubio’s opposition was rooted in politics, not court rulings. Mr. Rubio, a Republican, has seen his allure among conservatives tumble, a result of his aggressive push for immigration reform. In recent weeks, he has scarcely mentioned immigration, keeping his focus mostly on issues with broad conservative appeal like abortion and health care.