Facing the most perilous passage of his presidency, Barack Obama is to redouble his efforts to persuade a sceptical US Congress and American public to back strikes against Syria.

The President and his team are using a variety of methods to convince his opponents – among them videos of squirming gas victims, an Oval Office address and deploying myriad surrogates to speak on his behalf.

The task has in recent days become more daunting. Members of Congress return to Washington on Monday after mostly being besieged by their constituents to vote against action. Getting an authorising resolution through the Senate looks difficult; prospects of passage by the House look even tighter, with more than 200 members already indicating their opposition.

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What happens on Capitol Hill in the next days may determine whether missiles aimed at Syria are fired and also shape the legacy of Mr Obama, whose authority, at home and abroad, is surely on the line. While the President could theoretically ignore a “no” vote in Congress and order strikes anyway, to do so would almost surely elicit an impeachment effort by conservative and Tea Party Republicans.

To his critics, Mr Obama has only himself to blame for the predicament. “This is an unmitigated disaster. It’s amateur hour at the White House,” Karl Rove, the former Bush aide, declared last night. “If he gets the authority it shows that he’s not a lame duck,” said John Feehery, a former House Republican leadership aide. “If he doesn’t get the authority, it’s devastating.”

Denis McDonough, the Chief of Staff, played down the political stakes. “The President is not interested in the politics of this,” he said. Mr Obama will undertake some of the “heavy lifting”, as the White House now calls it, himself, sitting down with all the major networks and news cable channels on Monday to make his case before his Oval Office address on Tuesday.

But he will not be alone. The American Israel Public Affairs Committee has plans to swarm Capitol Hill tomorrow with 250 lobbyists to urge members of Congress to support strikes.

Israel’s Ynet news agency on Sunday evening quoted senior Israelis close to Prime Minister Benjamin Netanyahu as saying that in recent days he has spoken with members of congress and figures in the America Israel Public Affairs Committee, AIPAC, the lobbying group, to explain to them the importance of an American military action against the Assad regime.

Netanyahu’s spokesman, Mark Regev, declined to comment on the report.

Meanwhile the White House is recruiting figures of influence from every corner, including from past Republican administrations, to help to make the case. Hillary Clinton will speak out for strikes at two events this week. David Petraeus, a former general and CIA director, urged action, as did the former Defence Secretary Robert Gates.

Mr Obama received little help from his two days at the G20 summit in Stockholm and Americans awoke on Sunday morning to images of 100,000 people, led by Pope Francis, holding a vigil for peace in Rome. Then, on the CBS network on Sunday, President Bashar al-Assad surfaced asserting his innocence. “There has been no evidence that I used chemical weapons against my own people,” the network reported him claiming in an interview conducted in Damascus.

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In a sign that continued talk of military intervention may be having an effect on the Syrian leadership, Charlie Rose, who conducted the interview, said that Mr Assad expressed concern that a US attack might degrade the Syrian military and tip the balance in the civil war.

Meanwhile, John Kerry, the Secretary of State, arrived in London on Sunday as part of a tour of European capitals to stiffen the support of key allies. Speaking in Paris earlier, Mr Kerry said that Mr Obama has not ruled out further discussion of Syria at the UN Security Council, an option the French opposition is pressing on President François Hollande. “All of us are listening carefully to all of our friends,” he said. “No decision has been made by the President.”

Ahead of talks with Mr Kerry, William Hague, the Foreign Secretary, indicated that a second vote in Parliament might not be out of the question. “If circumstances change dramatically, then of course everybody would be looking at things in a different light,” he said on The Andrew Marr Show on Sunday. But he said the Government was not “gung-ho” about strikes, acknowledging “a lot of public unease about intervention”.

So far Washington has not seen any point in re-engaging the UN because of Russia’s veto or even awaiting the results of the UN inspectors’ mission to Syria. But the British ambassador to the UN, Sir Mark Lyall Grant, hinted debate may yet resume in the Security Council.

Hoping to push the debate in the President’s direction, the Senate Intelligence Committee yesterday posted 13 videos on the internet that “claim to show victims of a chemical or poison-gas attack”, according to the committee’s website. The videos, allegedly showing civilians gasping and suffering spasms after exposure to the attacks, were supplied by the Syrian opposition and assembled by the CIA.

“Those videos make it clear to people that these are human beings, children, parents, being affected in ways that are unacceptable,” Mr Kerry said. “And the US has always stood with others to say we will not allow this.”