Preet Bharara, the former United States attorney in Manhattan who was fired by President Trump, has registered as a Democratic voter in New York for the first time in more than a decade amid speculation that he might run for attorney general.

Mr. Bharara’s registration in Westchester County occurred during the window for candidates to gather signatures to qualify for the Democratic primary in September. A candidate needs thousands of signatures across half the congressional districts by mid-July — meaning time is short for Mr. Bharara to jump in.

Mr. Bharara did not immediately respond to a request for comment.

The Democratic primary for attorney general is already crowded. It includes Letitia James, the New York City public advocate, who was nominated at the state party convention; Representative Sean Patrick Maloney of the Hudson Valley; Zephyr Teachout, a law professor and former candidate for governor; and Leecia Eve, a former adviser to Hillary Clinton and Gov. Andrew M. Cuomo.

They are running to replace Eric Schneiderman, who resigned suddenly last month after multiple women accused him of physical abuse. Barbara D. Underwood, who was chosen last month to replace Mr. Schneiderman, has said she would not seek election in November.