The focus of the state's special U.S. Senate race is shifting to western Massachusetts this week. On Tuesday, Republican Gabriel Gomez and Democrat Edward Markey face off in Springfield for a debate that is expected to include questions key to the western region of the state. The match-up will be the second for the two candidates, who are hoping to fill the Senate seat left vacant by the resignation of John Kerry. A final debate sponsored by a consortium of Boston media outlets will be held on June 18. The election is June 25.