Donald Trump interrupted his 45-minute speech in Pennsylvania three times after his supporters began feeling unwell under the beams of searchlights.
The US president then called on the doctors to assist; one woman has been hospitalised.
According to journalists, Trump supporters had arrived for the rally seven hours before it actually started.
Donald Trump arrived in Montoursville, Pennsylvania to meet his supporters, two days after 2020 Democratic presidential candidate and former Vice President Joe Biden held a rally in the state's largest city, Philadelphia.
The rally came a day prior to a special election, where Trump's ally Republican Fred Keller will face Democrat Marc Friedenberg.