Clint Dempsey scored twice as the Seattle Sounders kept alive their dream of retaining the MLS Cup on Thursday with a 2-0 victory over the Vancouver Whitecaps.

Dempsey, who had missed the first leg of the Western Conference semi-final through suspension, pounced for two second-half goals as Seattle advanced to a meeting with either Portland or Houston.

Former Fulham and Tottenham striker Dempsey bagged his first on 56 minutes, calmly wrongfooting the Vancouver defense on the edge of the area before curling a left-foot shot past Stefan Marinovic.

The 34-year-old added a second two minutes from full-time, timing his run to perfection to pop up at the far post to bundle in Victor Rodriguez's cross.

Seattle are aiming to become the first team to retain the MLS Cup since Los Angeles Galaxy won back-to-back titles in 2011 and 2012.

Dempsey's double cheered a crowd of 39,587 who braved steady rain to pack into CenturyLink Field for Thursday's second leg.

Seattle dominated the early exchanges and might have had a penalty when Vancouver fullback Jake Newinski hauled down Nouhou Tolo in the first half.

Referee Baldomero Toledo studied a replay of the incident on a television monitor before deciding against a spot-kick.

Dempsey broke the deadlock early in the second half, completing neat inter-play between Harry Shipp and Will Bruin to curl home his finish.

The second came after captain Nicolas Lodeiro played a short corner to Rodriguez who found Dempsey at the far post.