Addressing a drive-in rally in Miami on Democratic presidential nominee Joe Biden‘s behalf, Mr Obama said the Republican president suggested he may “declare victory before all the votes are counted tomorrow.”
“That’s something a two-bit dictator does,” Mr Obama said. “If you believe in democracy, you want every vote counted.”
Mr Obama said if a Democrat was acting like Mr Trump, “I couldn’t support him.”
The former president said that, unlike Mr Trump, “With Joe and Kamala you’re not going to have to think about them every single day.”
“You’re not going to have to argue about some crazy tweet that the president sent out this morning,” Mr Obama said.
“It won’t be so exhausting. You’ll be able to about your lives know that the president’s doing his job instead of suggesting we inject bleach.”
On Sunday, a report by news site Axios said Mr Trump would declare victory on Tuesday night if it looked as if he was ahead.
Mr Trump denied the report, but said that counting ballots after election day was a “terrible thing”.
Officials warned that voters may have to wait days or even weeks for the result of the election due to an expected surge in postal ballots.
The last time the result was not clear within a few hours was in 2000, when the winner was not confirmed until a Supreme Court ruling was made a month later.
Different states have different rules on how and when to count postal ballots and in some states it will take weeks to get complete results.