About three dozen people fell ill at President Obama's campaign rally at Bowie State University in Bowie, Md., Thursday, WTOP radio reported.
The individuals passed out after complaining of "dizziness and fainting," Prince George's County fire department spokesman Mark Brady said.
Two people were taken to the hospital while the rest were treated at the scene, Brady told the radio station.
Officials were forced to set up a triage area at Bowie State's gymnasium to care for those who became sick.
Several of the victims got sick as they stood for several hours in anticipation of Obama, who came on stage just a few minutes later than his scheduled 3:15 pm ET arrival to stump for Democratic Gov. Martin O'Malley.
Weather forecasts had called for 77 degree heat in the late afternoon.
Approximately half an hour into the president's remarks, another audience member swooned, briefly derailing Obama's criticisms of Republicans.
He leaned away from the microphone. "Can we get a medic up here?"
Behind Obama, his well-orchestrated backdrop of smiling voters folded up their O'Malley signs and began fanning themselves and those around them.
O'Malley is in a tough fight for re-election Nov. 2 against former Republican Gov. Ehrlich.