Trump was introduced to the stage by Melania in an unannounced appearance.
The race for the White House remained essentially a dead heat on Sunday -- with nine days to go until Election Day.
Former President Donald Trump delivered a speech Sunday afternoon at New York's Madison Square Garden. After making several stops in Philadelphia on Sunday, Vice President Kamala Harris held a rally there Sunday afternoon.
Almost 42 million Americans have cast a vote through early voting methods, as of Sunday afternoon, according to data from the University of Florida's Election Lab.
The about 41.9 million recorded mail and early in-person votes were evenly split with about 21 million mail ballots returned nationally and about the same cast at in-person early voting polling sites across the country, the data showed.
Early voting options are now open to voters in 50 states and the District of Columbia. Many early voting periods will last until the weekend before Election Day.
As of Friday morning, more than 31 million Americans had voted early, according to the Election Lab at the University of Florida.
Of the total early votes numbering 31,402,309, in-person early votes accounted for 13,687,197 ballots and mail-in ballots numbered 17,715,831.
This means that more than 16 million people have voted since Monday.
-ABC News' Emily Chang
