The Bush administration and GOP leaders on Thursday renewed their push for Senate approval of John Bolton as U.N. ambassador. Democrats maintained he is too brash and ineffective to be confirmed.The sharp division all but guaranteed that lawmakers were headed toward another partisan showdown in the full Senate, although Democrats would not say whether their opposition would amount to a filibuster, as it did last year. A Senate vote on Bolton could come as early as September, just as election season heats up with Bush's foreign policy a major issue for voters. The United Nations has been at the forefront of international discussions on North Korea's missile tests, Iran's nuclear program and the crisis in the Middle East.