Although they expect George Bush to romp to reelection, Republican strategists are growing increasingly worried about losses in the Senate. Their latest cause for concern: a Nov. 5 special election to fill the GOP seat of the late John H. Heinz. One recent poll shows that former Attorney General Richard L. Thornburgh's once commanding lead over Democrat Harris L. Wofford is down to a 50-38 margin. Thornburgh, a two-term governor of Pennsylvania, is well ahead in fund-raising. But GOP pols worry that his heart really isn't in the race. Thornburgh has been lackluster on the stump and has been spending far too much time in Washington. "These are the kinds of races we worry about," says a senior Republican operative.