The New York Times has endorsed Democrat Hillary Clinton for the White House, saying she is more qualified than Republican presidential rival Donald Trump to handle the challenges facing the US.

The newspaper on Saturday described Clinton as "one of the most tenacious politicians of her generation" and said she had displayed a command of policy and diplomatic nuance while building a reputation for grit and bipartisan co- operation.

"A lifetime's commitment to solving problems in the real world qualifies Hillary Clinton for this job, and the country should put her to work," the Times said of the former secretary of state and US senator from New York.

Clinton will face off against Trump on Monday night in the first of three presidential debates, with opinion polls showing her once sizeable lead over the New York businessman narrowing amid continued public doubts about her trustworthiness.

The Times said Clinton's mistakes had distorted perceptions of her character, but praised her work restoring US credibility in foreign affairs as secretary of state and on behalf of children, women and families throughout her career.

"Mrs Clinton has shown herself to be a realist who believes America cannot simply withdraw behind oceans and walls, but must engage confidently in the world to protect its interests and be true to its values," the newspaper said.

It said Clinton's decision to use a private email server for government work as secretary of state deserved the scrutiny it has received in the campaign, but considered alongside the real challenges facing the US it "looks like a matter for the help desk".