PORTLAND (AP) — Clerks slogged through a mountain of ballots Oregonians turned in at the last minute, but the outcome of the U.S. Senate race between challenger Jeff Merkley and incumbent Gordon Smith remains too close to call.

Late today, only about three-fourths of the expected vote had been counted and the two candidates had swapped narrow leads.

That raised the possibility the outcome of one of the nation’s most-watched Senate races might not be known until Thursday.

The counting in Multnomah County was going particularly slow.

Merkley, the Oregon House speaker, is a Democrat. So he was expected to pick up strength as the returns came in from Multnomah, which includes Portland. It is the state’s most populous county and a liberal bastion.

In an interview late Wednesday, Merkley said he felt good about where he stood in the voting and was hopeful he would be declared the winner on Thursday.

“I am very optimistic about the outcome. I’m looking forward to having it resolved,” Merkley said.

Smith kept a low profile Wednesday as his camp waited to see whether the still-to-be-tallied ballots might turn things in his favor.

“Our campaign remains cautiously optimistic and eagerly awaits the counting of ballots,” said Brooks Kochvar, manager of Smith’s re-election campaign.

Multnomah County elections officials said their count could last well into the evening.

The long finish followed a tough race between Merkley and the two-term Republican incumbent Smith. It was the most expensive race Oregon has ever seen, with total spending estimated at more than $40 million.

Nationally, as Democrats raised their majority in the Senate to 56 of the 100 members, three other races remain unresolved. A runoff election is to be held in Georgia, a recount is scheduled in Minnesota, and the outcome in Alaska wasn’t clear.