Things are beginning to look up for Democratic incumbents as the party fights to retain control of the Senate this fall. In Virginia, Charles Robb is the apparent beneficiary of a withering attack by the national GOP Establishment on the character and credibility of his likely opponent, Iran-contra figure Oliver North. The state convention, dominated by followers of TV evangelist Pat Robertson, will pick the nominee, and North is expected to prevail over former Reagan Budget Director James Miller. Robb, whose job rating is below 50%, leads North 53% to 37% in a Washington Post poll. In Massachusetts, meanwhile, the GOP thought Edward M. Kennedy might be gettable this time. But he leads the Republicans vying to run against him--Janet Jeghelian, Mitt Romney, John Lakian, and Gary Todd--by margins of 31 points to 45 points in Boston Globe trial heats. And Republicans might want to rethink plans to recruit former U.S. Trade Representative Bill Brock to challenge Senator Paul Sarbanes of Maryland. A Mason-Dixon poll gives the three-term veteran a 43% to 22% lead. The Democrats now have a 56 to 44 majority in the Senate.