President Obama is back on the campaign trail in an effort to bolster support for Obamacare — a need reinforced by one Democratic senator's defection on the issue on Thursday.

Speaking at Prince George’s Community College in Maryland, Obama hammered on the need to enroll and endorse the program. "Part of the reason I need your help to make this law work," the president told the crowd of mostly young people, "is because there are so many people out there working to make it fail." As we've noted, the success of the policy depends on getting young, healthy people to sign up for (and pay into) the insurance exchanges that open their doors on October 1st. By January 1st, those young people (and everyone else) needs to be covered through an exchange, an employer, or a family member, or there's a penalty. (In 2014, that's $95.) Obama needs young people to sign up to make Obamacare (technically, the Affordable Care Act) work over the long term — but he also needs people to come to its defense in a tough political moment.

To his point, lots of people are working against the bill's success. Pressure on Obamacare has ramped up significantly over the summer, as the October 1st date approaches. Over the last two weeks, "Tea Party Republicans" (in the president's formulation) have tried to link a defunding of the policy — something the House has voted to do over 40 times since 2011 — to key government funding and debt decisions. Seizing on a July decision by the administration to delay an aspect of the policy for a year, conservative activists and people on Capitol Hill have called for that individual mandate that kicks in next year to be similarly delayed.

On Thursday, that effort gained a significant ally. The speech came close on the heels of Sen. Joe Manchin of West Virginia's announcement that he would support a short-term funding plan that included that sort of delay. Manchin, who represents a state that is predominantly Republican, told Bloomberg, "It's very reasonable and sensible." Manchin's position puts new pressure on his Democratic colleagues in similarly challenging states, like Mark Pryor of Arkansas and Mary Landrieu of Louisiana, both of whom are up for reelection next year. If the fight comes down to a filibuster in the Senate, Obama will need to hold every Democrat he can.

Which is why he gave that speech. "You're going to be the best, most credible messengers," he told the audience, pointing them to the site Healthcare.gov and encouraging them to get out and spread the word about the benefits of the program. The policy has polled increasingly negatively over the past few months, although the description of the bill affects the results. Obama himself, a The New York Times reports, is at a two-year low in his popularity.

Those low poll numbers make it harder for people like Manchin to stand in strong support of the president and the bill. Having seen what public opinion did to his Syria plans, Obama's undoubtedly feeling a renewed sense of urgency to provide political cover to other Democrats.

While making the case for signing up for the program, the president rebutted other critiques. "There's no widespread evidence that the Affordable Care Act is hurting jobs," he argued. He quoted a former economic advisor to the McCain campaign. "'I was expecting to see [job losses], I was looking for it, but it's not there. It's not there.'" This is aimed at rebutting a common critique from opponents, including a claim made repeatedly by Sen. Ted Cruz during his epic quasi-filibuster. He also pointed out that, like any large technical system, there would be glitches at launch. This was in part a response to a Wall Street Journal report on Thursday — but also meant to set expectations for news that broke shortly after the speech: two enrollment websites wouldn't come online right on Oct. 1.

Obama's speech was more energetic than he's seemed in recent weeks, almost challenging those Republican opponents. Perhaps his best line came near the end of the speech, suggesting that he would win the fight. "Once it's working really well, I guarantee you: They will not call it Obamacare." He argued that Republicans want the policy to fail because of the politics, not because of the policy.

The Republicans' biggest fear at this point is not that Affordable Care Act will fail. What they're worried about is it's going to succeed. Think about it. If it was as bad as they said it was going to be, they could just go ahead and let it happen and, man, everybody would hate it so much and then vote to repeal it and that would be the end of it. So what is it that they're so scared about? They have made such a big political issue out of this, trying to scare everybody with lies about death panels and killing granny, right? I mean, Armageddon. So if it actually works, they'll look pretty bad.

Unless the president turns around those politics and that public opinion, and quickly, more Democrats may be tempted to cave on allowing a delay of the program. And that delay would both throw the program into turmoil and allow his Republican opponents more time to try and undermine it. If the policy is killed, they will not call it Obamacare either. No one except political science professors will talk about it at all.