US President Joe Biden on Thursday closed a three-day NATO summit in Washington by promising to “keep NATO strong,” in a press conference that focused heavily on US politics and concerns about his mental fitness. The 81-year-old US leader is facing calls to let another candidate from his Democratic Party replace him in this year’s presidential race due to fears that he is suffering a mental decline. The opposing Republican Party is expected to nominate former president Donald Trump, 78, at its convention, which begins on Monday in Milwaukee, Wisconsin. “My predecessor has made it clear he has …