Murray clung to a wider but still small lead after the latest vote counts, with hundreds of thousands of ballots still to tally. Neither side had declared an end to the race.
With roughly two-thirds of the expected ballots counted by Wednesday night, Murray led Rossi by about 51 percent to 49 percent. The actual vote totals were 828,276 for Murray to 800,812 for Rossi — a margin of about 27,000 votes out of more than 1.6 million counted so far in unofficial returns.
"We're feeling good. We're feeling confident," deputy Murray campaign manager Alex Glass said Wednesday. [See who is giving money to Murray's campaign.]
"There's a number of ballots to be counted and we should make sure that every vote is counted," Rossi spokeswoman Jennifer Morris said.
Nearly everyone in Washington votes by mail and ballots may be returned through Election Day, leaving several days of processing and counting as envelopes postmarked as late as midnight Tuesday make their way to election officials.
The ballot-counting lag also means it's still unclear just how many people voted in the 2010 election. Election officials expected a 66 percent statewide turnout, which would be about 2.4 million voters.
Much depends on the results in heavily Democratic King County, which includes Seattle. Nearly a third of the state's roughly 3.6 million registered voters live in King County and Murray was maintaining about 62 percent of the county's vote in Wednesday's returns.
The national stakes for the Murray-Rossi race declined considerably once Democratic victories in West Virginia and Nevada ensured the GOP could not take control of the Senate. But a Murray victory would give Democrats a further cushion as Republicans take control of the House for the rest of President Barack Obama's term.
Obama made two visits to Washington to stump for Murray, including a raucous October rally attended by about 10,000 at the University of Washington. It was part of a push by national Democrats to turn out friendly voters here: former President Bill Clinton, Vice President Joe Biden and first lady Michelle Obama also visited Washington just as voters were receiving their ballots in mid-October.
State law would mandate a re-count if the race's margin is less than 2,000 votes. Washington has seen close races recently, including 2004's gubernatorial contest and Democrat Maria Cantwell's re-count victory over Republican Sen. Slade Gorton in 2000.
Murray's campaign for a fourth term featured a strong defense of her role in securing federal spending for Washington, highlighting the specific bridges, dams, highways, hospitals and construction jobs she helped deliver.
Rossi tried to turn those points against Murray, campaigning as a fiscal conservative who would forgo pet-project spending until the budget was balanced. He also criticized her support for Democratic overhauls of health care and Wall Street regulations.
Murray spent nearly $15 million through September to Rossi's roughly $2.5 million, but a flood of outside money helped Rossi keep up. Figures compiled by the Sunlight Foundation showed Rossi benefited from about $11 million in independent spending, compared with about $8.5 million in Murray's column.