WASHINGTON — Congress' deep reservations about President Barack Obama's push to train and arm vetted Syrian rebels were on full display Thursday as Democrats and Republicans on a Senate panel grudgingly backed the administration's plan.The Appropriations Committee endorsed Obama's proposal to spend $500 million for a Pentagon-run program that would broaden previous covert operations to equip rebels fighting forces loyal to Syrian President Bashar Assad and Sunni extremists. The money is contained in a $549.3 billion defense spending bill for the fiscal year beginning Oct. 1.In a challenge to the administration, Democratic Sen. Mark Pryor of Arkansas had offered an amendment to the bill that would have negated the program and transferred the money to overall spending on counterterrorism.Pryor questioned whether the administration can determine the moderate forces in Syria as the 3-year-old civil war grinds on with 170,000 dead. He said it would be impossible to keep track of weapons that could further destabilize the region and expressed doubts about an open-ended authorization to the administration."Our friends today could be our enemies tomorrow," Pryor said.As a reminder to his colleagues, he pointed to the current situation in Iraq where Iraqi security forces are struggling against the Islamic State extremist group after years of U.S. training and military equipment.Sen. Dick Durbin, D-Ill., chairman of the Appropriations defense subcommittee, acknowledged that the proposal was a gamble, but warned that if the U.S. didn't do it, it likely would do little or nothing."We are trying to stop Assad and the march of terrorism," Durbin said.Sen. Lindsey Graham, R-S.C., said it was "probably too little, too late" but a step that should be taken. Sen. Susan Collins, R-Maine, said the administration had failed to enunciate its goals for how it would spend the $500 million.The chairwoman of the full committee, Sen. Barbara Mikulski, D-Md., expressed strong ambivalence about the proposal.In the end, the committee rejected Pryor's amendment on a 21-9 vote, but he had the support of Democrats and Republicans, including Collins, Budget Committee Chairwoman Patty Murray, D-Wash., and Republican Sen. John Boozman of Arkansas.The Senate Armed Services Committee, in its sweeping policy bill, has backed arming and training Syrian rebels.The committee approved the spending bill that the full Senate will have to debate and decide. The legislation would provide $489.6 billion in core defense spending and $58.3 billion for operations in Afghanistan and other conflicts.Amid the latest hostilities between Israel and Hamas, the measure would double the amount of money for Israel's Iron Dome missile defense system, providing $351 million for the system that intercepts short-range rockets and mortars. Iron Dome has largely been successful in shooting down rockets.Since 1988 and the early days of U.S.-Israeli cooperation on missile defense, presidents have proposed a specific amount for the program knowing full well that members of Congress will increase the funds, especially after they hear from Israel.The bill spares several of the weapons programs that the Pentagon had proposed scaling back or eliminating in a cost-cutting move, including the A-10 Warthog, the close air support aircraft, and the USS George Washington aircraft carrier.Durbin emphasized research and development within the military. The bill would provide $25 million to fund a competition to develop a new domestic rocket engine. Durbin said reliance on a Russian-built version made no sense.