WASHINGTON — Hillary Clinton is not only the best-known, but also the most favorably viewed of the potential presidential candidates from either of the two major parties, according to a Gallup poll released Thursday.

The poll was conducted last week, mostly after the news broke about Clinton’s use of a private e-mail account while she was secretary of state, but before the news conference in which she addressed the issue.

Roughly nine in 10 Americans said they knew enough about Clinton to have an opinion, and the poll found that 50 percent viewed her favorably, while 39 percent had a negative impression. On both counts, that put her in better position than any of the potential Republican candidates for president.

Clinton’s favorability has declined since she left the State Department, as Americans have begun to see her as a presidential candidate, rather than in the more nonpolitical role of the nation’s top diplomat. When she left the agency, about two-thirds of Americans had a favorable view of her, a number that has dropped steadily as partisanship has taken its inexorable toll.