WASHINGTON — Just over half of Americans say they believe the interrogation methods the CIA used against terrorism suspects in the years after the Sept. 11, 2001, attacks were justified, polling data released Monday showed.

About 30 percent said they believed the tactics were unjustified, and the remaining 20 percent said they did not know, according to the survey by the Pew Research Center.

Opinion on the CIA’s torture of its prisoners differs notably by partisanship. Democrats were split, the poll found, with liberals much more likely to say that the CIA’s tactics were not justified. Republicans across the board said the interrogations were justified.

President Barack Obama banned the CIA’s use of methods such as waterboarding, extended sleep deprivation and beatings, which had been authorized under President George W. Bush. Obama and other Democratic elected officials have referred to the CIA’s actions as “torture.”

Most Republican elected officials have defended the CIA’s actions, with the exception of Sen. John McCain, R-Ariz., who was tortured as a prisoner of war in Vietnam and has denounced what the CIA did as a violation of American values.