With 66,452 watching on, Seattle roared ahead in the 25th minute with an own goal from Mamadou Danso, who in attempting to clear a cross from Christian Tiffert succeeded only in planting a volley past goalkeeper Donovan Ricketts.
Seattle pulled further clear of their derby rivals when Eddie Johnson swept a left-footed shot beyond Ricketts three minutes later, having been left unmarked by the penalty spot.
And the scoring was complete after 62 minutes when Freddy Montero tucked the ball in after a scramble around the edge of the Portland six-yard box.
Seattle, who still have a game in hand over all their Western Conference rivals, are three points behind second-placed Real Salt Lake and two clear of reigning MLS Cup champions Los Angeles.
The Columbus Crew could not obtain the unlikely win over Sporting Kansas City they needed to climb into the top five in the Eastern Conference.
They snatched a 1-1 draw against the Conference leaders though, with a 90th-minute effort from Eddie Gaven cancelling out a 10th-minute opener from CJ Sapong.
Columbus now stand one point adrift of the Houston Dynamo who hold the last available MLS Cup spot, while Kansas head the table by three points, with two games to play, from the Chicago Fire.
Chivas USA and FC Dallas also battled out a 1-1 draw, with Dallas forward Blas Perez netting in the eighth minute and Jorge Villafana replying in the 69th minute.
Chivas sit bottom of the Western Conference, while Dallas have only fading hopes of reaching the MLS Cup as they trail the Vancouver Whitecaps by four points.