A B737NG responded to a TCAS RA while on final approach to PDX.

Narrative

On visual approach to PDX runway 28L; inside outer marker; at app. 1;800'; talking to tower; cleared to land. Noticed traffic alert from TCAS. Advised pilot flying to be alert for possible traffic conflict. Traffic was on converging path at approximately the same altitude. TCAS Traffic Alert turned to Resolution Advisory (RA). Traffic was acquired visually at same time as RA received. Instructed pilot flying to immediately climb. Conflict was resolved when traffic passed under and behind us. The avoidance maneuver resulted in an unstable approach and we decided to execute a go-around/missed approach. Traffic was a small single engine; low-wing aircraft; ahead on our left side. The aircraft was approaching us on a converging path; and it seemed like it would cross the final approach path to RWY 28L at our approximate location; and at the same altitude. The traffic took no corrective action to avoid conflict; leading us to assume that he never saw us. I also think that the other aircraft must have been unaware of its proximity to PDX airport; since it was on a path to cross final approach to the active runway inside the Final Approach Fix at our approximate altitude.