Congressional Democrats say President Reagan's veto of a $299.6 billion military spending bill was desperation politics to help George Bush, a move that will backfire as the vice president seeks the White House. Angered by the Oval Office action, Democratic leaders have indicated there will be no attempt to override the veto. Instead they will add their spending authorizations to a defense appropriations bill, the legislation needed to put money in Pentagon accounts for the authorized military programs. The final result should be a package remarkably similar to the authorization measure Reagan vetoed Wednesday.