KAREN WALKER WASHINGTON D.C. In a bid that may be three-parts political and one-part desire, Continental Airlines has put in a bid to purchase DC Air, the startup carrier that US Airways and United Airlines promise to establish at Washington DC's National Airport as a sweetener to their proposed merger. US Airways, which is setting up DC Air using slots it owns at Washington National, says it will sell the startup to cable executive Robert Johnson, a director on the US Airways board. However, Continental's chief executive, Gordon Bethune, has sent a letter to US Airways and United offering $215 million in cash to purchase the slots and gate lease rights that would become the properties of DC Air. In a second letter, Bethune told the chief executives of US Airways and United that he was "disappointed" not to have received a reply to his offer. He pointed out that his offer was some 50% more than what Johnson will pay for DC Air.