Clear your desks, work your phones, get out and twist some arms.

That's the gist of the orders facing President Barack Obama's political team as they attempt to persuade wavering senators and congressmen to support the president's motion to mount a military operation against Syria. Yesterday, Secretary of State John Kerry kept up the momentum in Vilnius in Lithuania, where he spent the day trying to convince 16 EU foreign ministers to get onside and back the US.

At the same time, the rest of the White House team arrived back in Washington from the G20 Summit in St Petersburg with little or nothing accomplished on the diplomatic front. Today and tomorrow will be spent lobbying and on Tuesday evening, Obama will address the nation from the Oval Office.

Talking ahead of the announcement Obama said he would be attempting to "make the best case I can to the American people as well as to the international community for taking necessary and appropriate action".

When the vote is held in Congress in the middle of the week, it promises to be a tight call. Obama has already conceded it will be "a heavy lift" and he knows at least one-third of those voting are still undecided. In the next few days he has to convince them that a "limited narrow" strike will not lead to the mission creep that many US politicians fear will overwhelm them if Syria is attacked.

Those voting also know the rest of the world does not support a military intervention and they understand that it could plunge the US into a new Cold War relationship with Russia. Before Obama went to the G20 he had wanted to keep his contact with Russia to a bare minimum and had cancelled a planned summit with President Vladimir Putin to show his displeasure at the decision to grant political asylum to former US intelligence agent Edward Snowden. But as the summit came closer, US diplomats worked hard to engineer a meeting between the two leaders in an attempt to resolve the question of attacking Syria.

Despite a reportedly amicable "face-to-face" nothing came of the meeting and the two sides remain as far apart as ever.

While the US argued it had sufficient evidence to prove the regime of President Bashar al-Assad was responsible for last month's chemical attack near Damascus which left more than 1,400 people dead, the Russians countered that the evidence was not conclusive and more work needed to be done before a decision was made. By then, the rest of the world was also split on how to deal with the use of chemical weapons in the Syrian civil war and this left the US and Russia unlikely to combine in common cause over what should be done next. During the final press conference at the end of the G20 summit, it became apparent that neither Obama nor Putin were any closer to bridging the gap between their two countries.

While Obama put the case for military action in Syria, Putin said any such move would be "counter-productive" because it would destabilise the situation in the Middle East. In an attempt to lower the temperature, Putin revealed he and Obama stood their ground but at least there was dialogue. "We hear one another," he said, "and understand the arguments but we don't agree. I don't agree with his arguments, he doesn't agree with mine. But we hear them, try to analyse them."

As things stand, they are divided by the following three differences:

Evidence to prove who caused the chemical attack

The US has produced some evidence in the shape of communications intercepts and human intelligence from the ground in Syria which they believe provides solid proof the Assad regime either ordered or connived in the chemical attack. Much of this comes from their own resources but they have received help from Israeli intelligence services.

Russia believes the attack was arranged by rebel forces to create the present crisis. The Syrian government strenuously denies that it has ever used chemical weapons

Need for support of UN

From the outset Putin has warned action without UN approval would be "an aggression" and he is unlikely to budge. The Russians also support the view that no decision should be taken until the UN weapons inspectors have produced their findings, but this could take weeks and may not be conclusive. The Russians are supported by fellow G20 members China, South Africa, Indonesia, Argentina and Brazil

On the other hand, the US has rarely been prepared to acknowledge the primacy of the UN and believes its approval is unnecessary for military action to be taken against a country which they believe has broken international law.

Obama has said he does not feel bound by UN rules: "Given security council paralysis on this issue, if we are serious about upholding a ban on chemical weapons use then an international response is required, and that will not come through security council action."

Fear of losing influence in the Middle East

Both countries know their integrity in the Middle East is at stake. The US feels bound to show it means business by punishing Assad and in so doing sending a signal to Iran not to pursue its nuclear ambitions. Russia has made it clear that not only does it not support a strike against Syria but it could also lend Assad military support. At the conclusion of the St Petersburg summit he made his intentions clear: "Will we be helping Syria? We will. And we are already helping - we send arms, we co-operate in the economic sphere."

Both sides know everything it still to play for before any cruise missiles are fired against targets in Syria.