Former U.S. President Barack Obama waves while wearing a "Vote" mask as he campaigns on behalf of Democratic presidential nominee and his former Vice President Joe Biden in Philadelphia, Pennsylvania, U.S., October 21, 2020. Former President Barack Obama returned to the campaign trail on Wednesday with a blistering attack on Donald Trump with less than two weeks to go before the Republican president's Election Day face-off with Democratic nominee Joe Biden. REUTERS/Kevin Lamarque

Former U.S. President Barack Obama gestures as he campaigns on behalf of Democratic presidential nominee and his former Vice President Joe Biden in Philadelphia, Pennsylvania, U.S., October 21, 2020. With less than two weeks to go to the 2020 election, former U.S. President Barack Obama returned to the campaign trail in Philadelphia Wednesday, launching a blistering attack on Donald Trump and his handling of the health crisis. REUTERS/Kevin Lamarque

Former U.S. President Barack Obama speaks at a canvassing location as he campaigns on behalf of Democratic presidential nominee and his former Vice President Joe Biden in Philadelphia, Pennsylvania, U.S., October 21, 2020. Obama's support has been essential for Democratic candidate Joe Biden, helping his campaign outpace Trump in donations. REUTERS/Kevin Lamarque

Former U.S. President Barack Obama campaigns on behalf of Democratic presidential nominee and his former Vice President Joe Biden in Philadelphia, Pennsylvania, U.S., October 21, 2020. REUTERS/Kevin Lamarque

Former U.S. President Barack Obama speaks as he campaigns on behalf of Democratic presidential nominee and his former Vice President Joe Biden in Philadelphia, Pennsylvania, U.S., October 21, 2020. REUTERS/Kevin Lamarque

Former U.S. President Barack Obama speaks as he campaigns on behalf of Democratic presidential nominee and his former Vice President Joe Biden in Philadelphia, Pennsylvania, U.S., October 21, 2020. REUTERS/Kevin Lamarque