Wow I didn't know Washington used to have a solidly Dem congressional delegation. Was it Jackson's popularity that helped?
Not quite.
Congressmen like Lloyd Meeks (D, WA-2), Tom Foley, Floyd Hicks (D, WA-6) and Brock Adams (D, WA-7) were all able to defeat Republican incumbents during the LBJ wave of '64 and hold their seats for many more years to come.
On top of that, Henry Jackson's reelection coattails (getting 84% of the vote) and campaigning helped Democrat Mike McCormack defeat incumbent Cathy May of Washington's 4th. He was reelected 4 more times until he was defeated by Republican Sid Morrison in 1980, partly due to Reagan's coattails that year.