The crowd noticed it as the incumbent president wiped it off from his eye in front of a huge crowd in Iowa state.
Seemingly swept up in the significance of the event in Des Moines, the usually-stoic president appeared to weep from his left eye as he made his last plea to voters on the last night of campaigning, Daily Mail reported.
"I've come back to Iowa one more time to ask for your vote. This is where our movement for change began," Obama told the crowd of 20,000 people.
"You took this campaign and you made it your own. When the cynics said you couldn't, you said: 'Yes we can'."
Obama was joined on stage by First Lady Michelle, who hugged her husband and they happily clasped hands as they strode off stage.
The late night rally in Iowa was the final stop on the drawn out re-election campaign, and Obama capped off the last 24 hours with visits to Wisconsin, Ohio and Iowa.