Of nearly 2.9 million ballots cast, Coleman led Franken by 727 votes in unofficial returns from the Minnesota secretary of state. Coleman had 1,211,628 votes, or 42.03 percent; Franken had 1,210,901 votes, or 42.01 percent.
Dean Barkley of the Independence Party was third with 15 percent.
The margin was well within a threshold set by state law for an automatic recount that could drag into December. Franken had the option of waiving the recount, but he said he wouldn't.
"We won't know for a little while who won the race, but at the end of the day we will know the voice of the electorate is clearly heard," Franken said. "This has been a long campaign, but it is going to be a little longer before we have a winner."
Franken said his campaign was already looking into reports of irregularities in Minneapolis where some voters had trouble registering, though he wouldn't elaborate.
If the lead holds up, Coleman would be among the fortunate Republicans who survived big gains by Democrats nationwide. He planned an appearance later Wednesday.
Secretary of State Mark Ritchie, a Democrat, said a recount wouldn't begin until mid-November at the earliest and would probably stretch into December. It would involve local election officials from around the state.