A senior House Republican tells ABC News that if the vote to authorize a military strike in Syria were held today in the House, it would fail.

"He needs to make the case" the senior Republican said, adding that he believes the President can ultimately prevail but that he will need to convince a large number of Democrats to vote in favor of military action.

And on this vote there is no "Hastert rule" - Republicans say Speaker of the House John Boehner is committed to holding the vote even if a majority of Republicans are opposed to it.

President Obama yesterday said that although he believed he had the authority to order an attack on Syria without the authorization of Congress he had decided to seek their consent before ordering any such strike on the regime of Bashar al-Assad, which he accused of using chemical weapons last month against its own population, killing nearly 1500 people.

The president first talked to his chief of staff Denis McDonough about his desire to get congressional authorization prior to a strike late Friday.

The Map of Alleged Chemical Attacks in Syria

According to White House officials, the decision to seek Congressional approval was Obama's alone - none of his senior aides pushed for it - in fact they were surprised when he brought it up because it had not even been part of the discussion on what to do. Several top advisors to the president raised concerns, fearing the vote might fail.

But faced with the prospect of ordering military action not only without UN approval but without even the British, President Obama simply decided he did not want to act alone - he wants to put Congress on the record supporting his decision.

The congressional debate on authorizing the strike is expected to begin when members return to session on Sept. 9.

White House officials say that even though the president is seeking Congressional authorization, he does not rule out striking Syria even if he loses the vote in Congress.

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