President Obama asked Congress to delay a vote on whether to authorize military strikes against Syria, telling senators on Tuesday that the United States would not take military options off the table but would wait until a new round of diplomacy with Syria and Russia could play out.

With skepticism rising in Congress over Syria, the president spent more than two hours in rare closed-door meetings with senators. He said he was approaching the latest diplomatic effort with "hopeful skepticism," reprising a line from Ronald Reagan that he would "trust, but verify."

The president urged Congress to give him "more room," a blunt acknowledgement that the vocal opposition from Congress was creating deep complications and a failed vote would weaken the standing of the United States. He said it was critical that the threat of military strikes remain in place, or diplomacy would not succeed.

"His bottom-line ask of us was not to have us undercut his military power," Sen. Mark Kirk, R-Ill., said after his meeting with the president.

Pablo Martinez Monsivais/AP Photo

The first vote on Syria was set for Wednesday in the Senate, but Majority Leader Harry Reid called for a delay at the request of the White House. While administration officials conceded they did not have enough support, they also said diplomatic avenues needed to be pursued.

"If we're going to have any success diplomatically in the future on this issue, we have to make sure that the credible threat of military action remains," Reid said. "It's important to understand that the only reason Russia is seeking an alternative to military action is that the president of the United States has made it very clear that we will act if we must."

The president addressed the Democratic senators at their weekly policy luncheon first, taking questions for about an hour. Many senators in his party are skeptical of engaging in another military campaign in the Middle East, even a limited one, but the president implored them to reserve their opposition, at least for now.

"His basic message was this: With a threat of a military response, we'll have a much better chance with Syria and Russia," Sen. Tom Carper, D-Del., told reporters. "If we don't keep that threat open, they may very well walk away."

The president met separately with Republican senators. He entered the room to a round of respectful applause as he came face-to-face with some of his biggest conservative critics. But there were no confrontations with Sens. Rand Paul, Ted Cruz or others.

"I might have thought that some of the more conservative guys would take him on, but that didn't happen," Kirk said. "There was no political theater. It was generally responsible and statesmanlike."

While the president may not have immediately won over any critics or changed any minds, several Republican senators said the conversation was productive and they are willing to give him the time he asked for and not rush to a vote.

"The president is going to explore whether the Russian option is real. If it is, that would be good," said Sen. Lamar Alexander, R-Tenn., who had intended to oppose the Syria strike. "If there is a diplomatic solution that would remove chemical weapons from Syria that we could enforce that would be good for our country and it would be good for the world … we should know that in a week or two."

A congressional vote could be delayed for a week or longer.

"I don't think we need to rush out with our hair on fire right now. It's just not the thing to do," said Sen. Bob Corker, R-Tenn., who supports military strikes. "I think for a short period of time, our best course of action is to pause to understand whether this is credible or not."

The president was realistic in his assessment of the difficult politics surrounding the Syria debate, several senators told ABC News, and told them that he was willing to go against public opinion because of what he believes is a deep and grave threat from Syria's chemical weapons stockpiles.

"Polls are not going to change," Obama said during the Republican meeting, according to senators in the room. "I'm good, but not that good."

With opposition hardening against the original Syria resolution, several other alternatives have emerged. A bipartisan coalition of senators, led by Sen. John McCain and Sen. Chuck Schumer, are working to craft a backup plan.

Their resolution would give the United Nations time to take control of Syria's chemical weapons, aides told ABC News, and would require the removal of chemical weapons in Syria by a specific date. If that failed, the resolution would authorize military force.

Several other fallback proposals have also been floated on Capitol Hill, including from two Democratic senators, Joe Manchin of West Virginia and Heidi Heitkamp of North Dakota. They are proposing to give Syrian President Bashar al-Assad 45 days to comply with an international chemical weapons ban.

"I think the president was very receptive to that," Manchin told reporters. "But he understands that he wants to keep his finger on the pulse, if you will, and on the trigger if needed."