WASHINGTON — Congressional support for President Barack Obama’s push for a military strike against Syria continued to erode Monday as key lawmakers in both parties announced their opposition, while others began searching for alternatives to the proposed cruise-missile attacks.

As the opposition piled up, Senate Majority Leader Harry Reid, D-Nev., pulled back his plan for a test vote Wednesday on the use-of-force resolution, delaying by at least a day when that first hurdle could be cleared. This will allow Obama to make his case to both Senate caucuses at Tuesday’s weekly policy luncheons, as well to the nation in a televised address Tuesday night.

On Monday, a handful of senior Senate Republicans announced their opposition as other GOP leaders remained silent or openly skeptical of Obama’s plan, while another conservative Senate Democrat rejected the proposal outright. The combination of red-state Democrats and senior Republicans backing away from the Syria resolution fueled doubts about whether Obama could win a vote in the Senate, where victory had been considered more likely, given the chamber’s history of support for muscular foreign policy.

Opponents continued to emerge on both sides of the aisle in the House, where public support among lawmakers remains anemic.

Almost every lawmaker opposing strikes made clear that they did not question the administration’s case against Syrian President Bashar Assad’s regime — that he used sarin gas against civilians, killing more than 1,400 — but they remained unconvinced that the limited missile strikes that the president proposes would yield anything productive .

“After all these meetings, I still have serious concerns. I cannot support the current Senate resolution to authorize force at this time,” Sen. Heidi Heitkamp, D-N.D., a freshman elected in November in a state that Obama lost by more than 20 percentage points. “After doing my due diligence, I believe we need an alternative path forward in dealing with the Assad regime. We must balance the legitimate concerns that Americans have about the use of military force with our strategic interests.”

Republican Sens. Roy Blunt, Mo., and Lamar Alexander, Tenn., dealt a blow to the White House effort to build GOP support for a strike when they announced that they would oppose the plan. The decision of those two, more-mainstream conservatives within the GOP caucus is a significant setback for the president. Later, another set of establishment Republicans — Sens. Mike Enzi, Wyo., Johnny Isakson, Ga., and Roger Wicker, Miss. — announced their opposition.

Additionally, Sen. Susan Collins, R-Maine, a bellwether as the chamber’s leading GOP moderate, told reporters Monday evening that she was “leaning against” the resolution despite spending three hours at Vice President Joe Biden’s home Sunday night with a group of fellow Senate Republicans discussing the Syria issue. Obama joined the group for an hour.

Reid’s action on the Syria resolution came amid growing opposition and minimal support in the Senate.

Sen. Barbara Mikulski, D-Md., a leading liberal, led the charge with a rousing defense of Obama’s plan. “Let me be clear: I have no grand hopes or illusion about what this strike will do . . . but I do believe that it will deter and degrade [Assad’s] capacity to strike again,” Mikulski, previously undecided, announced in a Senate floor speech.

The administration, meanwhile, continued in full-press mode in an effort to win over wavering Democrats and Republicans, ushering dozens of lawmakers into the White House for meetings Monday and planning visits to the Capitol on Tuesday by Obama to make direct appeals to senators.

The meetings will take place hours before the president’s national address on the situation in Syria.

New efforts to find a diplomatic resolution to the Syria crisis are complicating the administration’s continuing bid to push lawmakers to support the military resolution. Obama supporters, including Sen. Benjamin Cardin, D-Md., and Rep. James Moran, D-Va., noted those new efforts, including a Russian bid to get its allies in Syria to turn over its stockpile of chemical weapons.

“I have concerns about action,” wrote Cardin on his Twitter feed. “Right now we need to deal with #Syria via diplomacy if possible.” He was among those on the Senate Foreign Relations Committee who voted last week in favor of using force.

Moran said he hoped that overtures Monday by Russian President Vladimir Putin “may be a game changer” that could help lawmakers avoid voting on the issue altogether.

“I think it’s a very important proposal, and I think it needs to get followed up very quickly,” Sen. Dianne Feinstein, D-Calif., chairman of the Select Committee on Intelligence, told reporters Monday afternoon.

Others viewed the Russia development as a red herring, particularly as it would leave Assad in place. Sen. Bob Corker, Tenn., the ranking Republican on the foreign relations panel, questioned how the administration would verify that the entire batch of Syria’s chemical weapons had been turned over to the United Nations.

Late Monday evening, Sen. John McCain, R-Ariz., convened a group just off the Senate floor that supported striking Syria, telling reporters that he was working on a proposal that would allow for Russia to work with Syria to get the regime to release its chemical weapons stockpile. According to a Democratic aide familiar with the emerging proposal, the bipartisan group would amend the use-of-force resolution to give the United Nations one week to work with the Russians. If the efforts were unsuccessful, Obama would have authority to order military strikes.

Six Senate Democrats have announced opposition to the Obama plan, while seven Republicans have announced support. Dozens remain undecided on both sides. With 54 members in the Senate Democratic caucus, Obama may have to rely on getting Democrats who oppose the strikes to at least agree to vote to cut off a filibuster, so that the resolution can get to a simple majority vote on final passage.

In addition to the White House, key outside groups are mobilizing to support the effort.

The influential American Israel Public Affairs Committee (AIPAC) is sending about 300 of its activists to Capitol Hill on Tuesday to meet with about 300 senators and House members and urge them to support the resolution.

An AIPAC official said the group is playing an active role because it sees a direct connection between the Syria crisis and Iran’s effort to get nuclear weapons. “If America is not resolute with Iran’s proxy Syria on using unconventional weapons, it will send the wrong message to Tehran about their effort to obtain unconventional weapons,” said the AIPAC official, who spoke on the condition of anonymity to talk frankly about the effort.