WASHINGTON — President Barack Obama on Thursday asked Congress for the first lethal aid to arm moderate Syrian rebels in a move aimed against both Syrian President Bashar Assad and radical Islamists who have increasingly dominated the opposition movement.Obama has urged helping moderate Syrian rebels for months, but his request for $500 million is the first time he’s put a dollar tag on the proposed aid. And it was the first time he specified that the money should be used, in part, to arm and train the rebels.The request was part of a larger $58.6 billion appropriations package for overseas wars, with most of the funds slated for Afghanistan. An Obama national security aide said the $500 million should be used to help Syrian rebels topple Assad, while at the same time defeating militants who call themselves the Islamic State of Iraq and Syria, or ISIS.“As we have said many times before, Assad has lost all legitimacy to rule Syria and must go,” Bernadette Meehan, a National Security Council spokeswoman, told McClatchy. “The request to Congress reflects our assessment of the time needed to launch such a program and our view that building the capacity of Syrians for stabilization and counterterrorism operations will be necessary both during the (current) conflict and after a negotiated settlement.”A White House fact sheet on the broader Overseas Contingency Operations budget request said Obama is seeking $500 million “for a proposed authority to train and equip vetted elements of the Syrian armed opposition.”In a speech last month to graduating cadets at the United States Military Academy in West Point, N.Y., Obama reiterated that he would not send U.S. troops to Syria.“But that does not mean we shouldn’t help the Syrian people stand up against a dictator who bombs and starves his own people,” Obama said. “And in helping those who fight for the right of all Syrians to choose their own future, we are also pushing back against the growing number of extremists who find safe haven in the chaos.” Despite Republican lawmakers’ criticism of Obama’s policies in the Middle East, it is far from certain that Congress would approve lethal aid to Syrian rebels _ even if they are, in the words used by the White House on Thursday, “vetted” and deemed to be “moderate.”Congress last September rejected Obama’s bid to launch airstrikes against Assad in retaliation for the use of chemical weapons, which the United States and key allies accused him of using in the country’s civil war. Since then, the security situation has become more muddled, with the violence spilling over into neighboring Iraq and threatening the U.S.-backed government of Prime Minister Nouri al Maliki.ISIS militants have stepped up attacks on government security forces in both countries while fighting pitched battles in Syria against other rebel groups.The Assad government Tuesday bombed ISIS fighters in Iraq, bolstering government security forces trying to repulse a two-week offensive that saw the militants capture key cities north of Baghdad and also gain footholds to the west and south of the capital.Sen. Carl Levin, D-Mich., chairman of the Senate Armed Services Committee, expressed support for Obama’s funding request “in light of recent events in Iraq and Syria.” He said the president’s bid “to train and equip vetted elements of the Syrian opposition” was similar to legislative language contained in the pending Defense Authorization Act and recently approved by a large bipartisan majority of the committee.In a Senate floor speech before Obama sent Congress his funding request, Sen. Lindsey Graham, R-S.C., who sits on the armed services panel, focused on the need to defeat the ISIS militants waging war in both countries.“Mr. President, do not use the political problems in Baghdad as an excuse not to act when it comes to denying safe havens to terrorists,” Graham said. “Where is your plan to dislodge these people in Syria and Iraq?”Even if Congress approves Obama’s request, there are serious questions about how much impact the $500 million would have more than three years into a Syrian civil war that has claimed an estimated 150,000 lives, uprooted more than 9 million people and laid waste to large parts of towns and cities.Another major rebel group is the Nusra Front, an al Qaida affiliate that has battled ISIS fighters even as it tries to overthrow Assad. While the Islamists are well-armed and organized, the U.S.-backed Free Syrian Army and other moderate groups have been unable to make major gains, in part because they lack unified military and political leaderships.If approved, the $500 million could be used to train and equip moderate Syrian rebels in Jordan, building on a modest CIA-run program that already has helped small numbers of anti-Assad fighters in the Free Syrian Army.At least 11 moderate groups in Syria in recent months have begun using U.S.-made TOW portable anti-tank missiles, weapons they likely could not have received without at least tacit U.S. approval.Jeffrey White, a former Defense Intelligence Agency analyst, said moderate Syrian rebels also could be trained and equipped in Turkey, which some rebels fighting in northern Syria already use as a sanctuary.“For $500 million, you should be able to get a pretty decent training program and buy a lot of weapons,” White told McClatchy. “What you are going to see is an intensified fight against the regime. The war in Syria would step up a level. We should see an intensified fight against ISIS as well.” David Lightman of the Washington Bureau contributed.