President Reagan framed his complaint against Michael Dukakis this way: "If he's Harry Truman, I'm Roger Rabbit." The quip, which Reagan slipped into the prepared text he brought to a rally in a Berea, Ohio, gymnasium, neatly summed up the case the president has been pressing against the Democratic nominee in a weeklong cross-country tour. Over and over, he took aim at Dukakis for saying he is the heir to the liberalism of Presidents Franklin D. Roosevelt, Harry S. Truman and John F. Kennedy. Instead, Reagan told the cheering audience in the Baldwin Wallace gym, the Massachusetts governor is the kind of liberal who is against gun control, against school prayer and for the "philosophy of tax and tax and spend and spend." That's when he threw in the Roger Rabbit line.