By Carlo Munoz and Jeremy Herb - 06/19/12 09:29 PM EDT
Those observers were on the ground to ensure Assad complied with a peace plan forged by U.S.-Arab League envoy in March.
The U.N. pullout has increased calls for U.S. and NATO-led military action against the regime, but Sen. Mark Begich (D-Alaska) said the U.N. decision has done little to move the needle toward military action among Senate Democrats.
Sen. Ben Nelson (D-Neb.) attempted to play down the U.N. decision to pull its forces from Syria, noting his conversations with regional allies Egypt and Saudi Arabia would hold more sway over congressional support for military action.
"That has had more influence on me than anything the United Nations may or may not be doing," Nelson said. "I at times find the United Nations helpful, and at times I do not find them helpful at all."
Sen. Carl Levin (D-Mich.), who chairs the Senate Armed Services Committee, noted the pullout had more of an effect on Assad's forces, indicating the window for a peaceful transition is beginning to close.
The withdrawal, according to Levin, represents "a signal to the Syrians that the options [now] are limited."
Sen. John Kerry (D-Mass.) said Tuesday the withdrawal was a sign the United States should double down on diplomatic and economic sanctions on Syria.
"I think we have to increase the pressure [and] everybody’s looking at what the menu ought to be," Kerry told The Hill.
Sen. Daniel Akaka (D-Hawaii) agreed with Kerry, saying now is not the time to abandon White House and its attempts to transition Assad out of office.
"We must make every effort to continue diplomatic solutions ... instead of getting into any conflict" with Syria, Akaka said. The U.N. decision, he added, has only reinforced the White House's resolve in finding a peaceful solution.
"We need to work harder," Akaka said.
The Obama administration was working Monday to secure support for a non-violent solution in Syria from Russian President Vladimir Putin.
Both leaders agreed that Washington and Moscow shared many “commonalities” on Syria, and Obama said they agreed there had to be a “cessation” of the violence, according to a joint statement issued Monday.
But the impact of those talks on the Syria conflict will become clearer when lawmakers are fully briefed on them, according to Sen. Dick Lugar (R-Ind.)
"We all [are] looking forward to those briefings," he told The Hill on Tuesday.
Those briefings, according to Blumenthal, could hold the key to whether Senate Democrats continue to toe the White House line.
When asked how lawmakers will know if time has run out for diplomacy in Syria, he replied: “When we know that the Russians ... are going to continue to be completely uncooperative as they have been.”