President Obama’s stunning reversal on Syria — deciding to ask Congress to approve the use of force just hours after he seemed set on bypassing the legislative branch — amounts to a massive gamble by the commander in chief.

There is little certainty of the outcome of the vote, which is expected to come the week of Sept. 9 when both houses of Congress return to Washington after the August recess. And, if Congress doesn’t pass the resolution, Obama will be in an even smaller box — policy-wise — than he found himself at the end of last week following the British Parliament’s rejection of a similar use of force resolution.

Let’s start by walking through just how big a challenge President Obama has built for himself.

First, consider that roughly 40 percent of House Democrats voted against the use of force resolution against Iraq in 2002. (Unlike 2002, Democrats have one of their own in the White House now, but the 2010 election made the caucus more liberal — and more opposed to military action — than it was in 2002.)

Second, remember that Obama is in the middle of his second term. He is playing for his legacy; all — or at least the vast majority — of the Democratic members he will ask to vote in favor of striking Syria are playing for the 2014 election. Those are two very different calculations — especially when you consider that many of the Democrats whom Obama will need are running in districts where the only real threat is from their ideological left. A vote for a controversial military action is perfect fodder for a liberal challenger looking for an issue that will take down a Democratic incumbent.

Third, Obama’s relationship with Congress — including those within his party — has never been all that great. He spent little time there during his own career, and Democratic House strategists have long believed that Obama is semi-openly disdainful of the people’s House. And, having a long-time Senate aide — Denis McDonough — as his chief of staff won’t help President Obama much in the House either. (The perfect chief of staff for this moment in the House is currently serving as the mayor of Chicago.)

Fourth, the shadow of Iraq looms. You can tell how much by listening to Secretary of State John Kerry make the case for action in Syria on Friday. “Our intelligence community has carefully reviewed and re-reviewed information regarding this attack,” Kerry said. “And I will tell you it has done so more than mindful of the Iraq experience. We will not repeat that moment.”

The question is whether Kerry’s testimony in front of House and Senate committees this week can convince lawmakers of that fact. And, because of how Iraq (and the lack of WMDs) played out, the hurdle is that much higher.

Fifth, the “why now/what now” question remains a tough one to answer for many members of Congress. Yes, use of chemical weapons is a clear line that has been crossed. But more than 120,000 Syrians have died since Obama first called on President Bashar al-Assad to step aside, and most foreign policy experts believe any strikes launched by the United States will be extremely narrow in both their scope and length.

Despite all of those factors arguing against passage, President Obama pushed forward for a vote — believing, according to behind-the-scenes reporting done by The Post’s Scott Wilson — that if he end-ran Congress on this issue he might lose any chance to work with them on things such as the looming government shutdown and the debt ceiling.

That makes sense if the resolution passes. But, if it fails and President Obama goes forward with a military action anyway — as administration officials have made quite clear they believe he can and might do — relations with Republicans in Congress (and, in truth, many Democrats) will be even more strained.

Republicans in Congress have long argued that the president is far more interested in using them as a political foil than in actually accomplishing things in a bipartisan matter. If he ignores a vote against the Syria resolution, the traces of trust that exist between the GOP majority and the White House will disappear — almost certain to not return in time for the government shutdown/debt ceiling fights.

Add it all up and it’s plain to see just how big a gamble President Obama is taking — and just how large the political stakes are for him if he loses his bet.