US President Barack Obama has aimed his economic message at female voters as he campaigns on the West Coast for candidates key to Democrats' chances of keeping their majority in the US Senate.

Obama will try to bolster incumbent Senators Patty Murray of Washington and Barbara Boxer of California with rallies and fund-raisers over the next two days, during the trip, his longest campaign swing as president.

"How well women do will help determine how well our families are doing as a whole," Obama told a gathering of about 30 people in a Seattle backyard before taking a series of friendly questions.

Polls have shown an erosion in the slight leads held by the two Democratic candidates to razor-thin margins, with less than two weeks left before the November 2 congressional elections.

Obama is seeking to rev up core Democratic voters and is targeting women, who vote Democratic in higher percentages than men, as part of that strategy.