Democrats must gain six seats in the November 7 election to recapture U.S. Senate control, and they lead in three of the seven most vulnerable Republican-held states, according to a Reuters/Zogby poll released on Thursday. Democrats lead Republican incumbents in Montana, Rhode Island and Pennsylvania. Republican incumbents lead in Virginia and Missouri, the polls found, with races in Republican-held Ohio and Tennessee deadlocked. The polls of at least 600 likely voters in each state have a margin of error of plus or minus 4 percentage points.