Speaking to supporters in Columbus, Ohio, the Illinois senator said despite his lead in the polls, his race against Republican John McCain is far from over, CNN reported.
"Don't believe for a second this election is over. Don't think for a minute that power will concede without a fight. We have to work like our future depends on it in these last two days, because it does," Obama said.
Obama also used his speech to hammer the Arizona senator for his ties to the Bush administration, noting Vice President Dick Cheney's endorsement of the Republican ticket.
"I'd like to congratulate Senator McCain on this endorsement, because he really earned it. That endorsement didn't come easy," Obama said.
The Obama camp Saturday released an ad that highlights Cheney's endorsement, which came Saturday.
A poll shows Obama leading McCain, 52 percent to 46 percent among likely voters in the battleground state of Ohio.
The mail survey by The Columbus Dispatch of 2,164 likely Ohio voters was conducted Oct. 22 through Friday, with a margin of sampling error of plus or minus 2 percentage points.