WASHINGTON — A July 25 call between President Trump and the president of Ukraine is the basis for an impeachment inquiry into whether Mr. Trump withheld American military aid until Ukrainian officials investigated former Vice President Joseph R. Biden Jr. and his son, Hunter.

Last week the acting White House chief of staff, Mick Mulvaney, effectively acknowledged the quid pro quo, although he said the aid was in part contingent on Ukraine’s investigating Mr. Trump’s widely debunked theory that Ukraine, not Russia, was responsible for hacking Democratic Party emails in 2016. The theory is politically helpful to Mr. Trump because it would show he was elected president without that Russian help.

Mr. Mulvaney was unapologetic in his remarks. “I have news for everybody: Get over it,” Mr. Mulvaney told reporters at the White House. “There’s going to be political influence in foreign policy.” (He later reversed himself and has said his comments were misconstrued.)

Readers have asked The New York Times to explain why, exactly, another nation’s interference in the democratic process is such a serious issue.