WASHINGTON - A defiant Democratic-controlled Senate passed legislation Thursday that would require the start of troop withdrawals from Iraq by Oct. 1, propelling Congress toward a historic veto showdown with President Bush on the war. The 51-46 vote was largely along party lines, and like House passage of the same bill a day earlier, fell far short of the two-thirds margin needed to overturn the president's threatened veto. Neverthe less, the legislation is the first binding challenge on the war that Democrats have managed to send to Bush since they reclaimed control of both houses of Congress in January.