AP Photo Democrat Jeff Merkley declared this morning he has won the U.S. Senate race, unseating incumbent Republican Sen. Gordon Smith.

PORTLAND - Democratic challenger Jeff Merkley has unseated two-term incumbent Republican Gordon Smith in a hard-fought U.S. Senate race.

It was the most expensive race in Oregon history, with spending estimated at more than $40 million. A flood of votes Oregonians delivered on election day kept election workers tallying ballots for two days.

The victory gives Democrats at least 57 votes in the U.S. Senate with the outcome of three other races yet to be determined. A runoff election is to be held in Georgia, a re-count is scheduled in Minnesota, and the outcome in Alaska wasn’t clear.

The statewide ballot count shows Merkley with 775,211 votes to 735,936 for Smith.

Smith plans a press conference this afternoon. Smith’s spokeswoman Lindsey Gilbride said she couldn’t say whether Smith would concede at the press conference he scheduled for 2 p.m.

Before a 9:30 a.m. appearance at the Hatfield School of Government at Portland State University, Merkley issued a statement saying he was “the first leader in 40 years to defeat an incumbent U.S. Senator in Oregon.”

Forty years ago this week, Republican Bob Packwood was declared the winner in an upset of Democratic Sen. Wayne Morse.

Early Thursday, about 81 percent of the vote had been counted statewide. Merkley had a margin of nearly 40,000 votes.

His victory margin came from his home county — Multnomah. He represented an east Portland district as speaker of the Oregon House.