WASHINGTON: Kamala Harris said Monday she would take a cognitive test in response to jabs from Donald Trump, and challenged the ex-president to take one too as mental fitness becomes a battle line in the White House race.

Harris has repeatedly said her 78-year-old Republican opponent, now the oldest presidential candidate in US history, is increasingly “unhinged” and unfit to lead the country again. Trump has meanwhile called the 60-year-old Democratic vice president stupid, attacking her IQ in his pitch to voters in a race that the polls suggest is effectively tied. Harris was asked in a CBS interview segment released Monday if she would take a cognitive test, as Trump had called for on social media.