President Obama, speaking at the G-20 meeting in St. Petersburg earlier today, reminded me of an investment banker trying to sell a deal he doesn’t believe in. The customer knows it.

Halting. Hesitant. Uncertain. Uncomfortable. That’s what Mr. Obama’s statement and body language had to say.

On the verge of a potentially huge defeat on the Syrian question in Congress, the President is in a box. He’s looking for a way out, but he can’t find one. He’s losing supporters in the legislature at home, and he didn’t gain any at the G-20 summit abroad.

When Mr. Obama speaks to the American people on Tuesday, maybe he’ll pull a rabbit out of the hat. But House and Senate members and their staffers on both the left and right say the grassroots do not want a Syrian bombing mission. I can’t speak for the Left. But I know conservatives do not trust the president in his role as commander in chief. They want more than a shot across the bow, but they’re not hearing clear strategies and intentions.

Worse, in recent days, the President has retreated back to terms like “limited,” “narrow,” “tailored,” and “negotiated.” He is reminding everyone that he has no intention of Assad regime change or of crippling the Syrian military.

Some news accounts imply a tougher mission that would take out the Syrian air force and airports, and cripple command and control along with any related chemical-weapons delivery systems. But this sounds more like Secretary of State Kerry, who has done a good job outlining the military and strategic imperatives of a tough attack. It’s not coming from President Obama.

At the G-20, it was Vladimir Putin who sounded tough as he pledged aid to Syria in the event of an American attack. That brings up the question of retaliation by Iran and Hezbollah. In that event, what will Israel do? Counterattacks have not been addressed. There’s no sense of what might happen next.

A congressional defeat on Syria might well be catastrophic for American credibility, as Senators McCain, and Graham and a former senator, Joseph Lieberman, suggest, and as Speaker Boehner and Majority Leader Cantor echo in the House. Everybody seems to know this except the president.

It’s a bad situation in and of itself. But spillover effects into economic and fiscal affairs will make it worse.

Take Friday’s jobs report. It came in well below expectations. It looks like the pace of job creation is slowing, meaning there will be no second-half economic upturn.

Only 152,000 private jobs were created in August, while June and July were revised down by 74,000 jobs. Measured over three-month periods, job creation has dropped to only 158,000 through August from 236,000 last February.

While unemployment dropped slightly for August, down to 7.3% from 7.4%, the participation rate fell again as more discouraged workers left the labor force. Adjusted for inflation, wage earnings have completely flattened out.

People are buying cars. The ISM reports are strong. But business investment in long-lived projects — the most powerful job-creator of all — is actually falling.

So it’s still an anemic 2% economy. When the president returns from overseas, he’ll get no plaudits for a new employment boom. More important, should Mr. Obama lose the congressional vote on Syria, his entire domestic agenda might fall apart.

At least three key Democratic senators on the banking committee are saying no to the possible nomination of Obama-favorite Larry Summers to the Federal Reserve. When the Fed itself meets on September 17 and 18, it will probably defer any widely advertised bond-purchase cutbacks until the Syrian crisis has passed.

Two weeks after that, unless a continuing resolution gets passed, the government shuts down. Two weeks after that, the nation’s borrowing power expires, making a higher debt ceiling necessary to keep operations going.

On top of all that, the military is screaming that they need more money, because the president is asking them to do more with less.

How can a president defeated on Syria hope to cope with all this? It’s almost beyond imagining. Which begs perhaps the largest question of all: Will a Syrian defeat in Congress cripple the administration completely, with 40 months left in the executive’s term?

None of this is as important as America’s global credibility when the U.S. must not stand back and let rogue dictators use weapons of mass destruction. That said, Mr. Obama’s foreign-policy blunders may end his domestic policy too.