Jesse Jackson could easily defeat District of Columbia Mayor Marion Barry Jr. if an election were held Wednesday, according to a poll in Wednesday's Washington Post. Jackson would win 68 percent of the vote to 17 percent for Barry. The Post's poll also found that Barry would lose to Police Chief Maurice A. Turner Jr., 60 percent to 24 percent, and to three other possible challengers. Barry has been facing declining support because of questionable behavior and mounting city problems.