Any notion that President Obama’s reelection campaign was gaining momentum was shaken this week by a string of worrisome economic reports showing weakness in the job market and new lows for housing prices.

The bad news for Obama stood in contrast to a run of positive developments that had given many Democrats reason for confidence. The economy had been adding jobs at a steady clip. A president once accused of being weak on national security ordered the raid that led to the killing of Osama bin Laden. And several potentially strong Republican presidential candidates took a pass on the race.

But on Friday, a surprisingly grim employment report was a warning that Obama could face a more challenging economy than had been expected. The government said the economy had added far fewer jobs than analysts had projected.

The disappointing news posed a predicament for Obama, who had scheduled a visit to a Chrysler plant Friday to herald the rebirth of the U.S. auto industry. As he gave a celebratory speech to cheering autoworkers, his advisers in Washington scrambled to explain the jobs report.

During Friday’s visit to the plant in Toledo, Obama did not mention the grim jobs number that was released Friday. Rather, he talked of saving a “proud industry” and “more than a million jobs.”

Still, he acknowledged continuing challenges. “[W]e still face some tough times,” Obama said. “There are still some head winds that are coming at us.”

Some political analysts said Obama would have to confront just how fierce those winds are.

“The prospect of economic growth getting up to a point and unemployment getting down to a point that is comfortable for an incumbent are declining by the month, and are now not very high at all,” said William Galston, a policy adviser in the Clinton White House and a veteran of Democratic presidential campaigns.

He added, “I hope there’s someone on the inner circle with the standing and the guts to tell the president that, if things continue the way they’re going, despite everything he’s done, he’s going to be in trouble.”

Behind the economic distress is a series of unexpected events, including the earthquake and tsunami in Japan, the European debt crisis and rising gasoline prices. As a result of the unemployment rate turning back up and the housing market reaching new lows since the slump began in 2006, numerous economists have reduced their expectations for economic growth this year.

Even more challenging for Obama is that some of the hardest-hit states, such as Florida, Nevada and Michigan, are critically important for his reelection strategy.

Democrats believe the auto bailout can be a big political winner in the industrial Midwest. But the administration has struggled to show progress in helping struggling homeowners.

And White House allies concede that the economy may present greater challenges than they had thought.

“The president is going to be running for reelection in an economy that’s still too weak,” said Jared Bernstein, who until last month was chief economic adviser to Vice President Biden. “It is improving and is in a far better place than it was when he got there but still is not adequately lifting the living standards of the broad middle class.”

Guy Molyneux, a Democratic pollster, said that while the economy might not be the sole factor in deciding Obama’s political fate, it will set the overall frame of the campaign.

“If things get worse, it would take a great campaign or a terrible failure by his opponent for Obama to win,” Molyneux said. “And if this ends up being just a hiccup and we see strong growth next year, then a Republican victory starts to look pretty unlikely.”

Obama’s advisers said Friday’s job report could be an aberration and should not distract from the president’s success in helping rescue the economy from the worst recession since the Great Depression.

Dan Pfeiffer, the White House communications director, said on MSNBC that the plant Obama visited would have been closed if any of the Republican candidates had been president.

The jobs report was used in sharp attacks from leading Republican presidential candidates.

“What [Obama] did simply was wrong,” former Massachusetts governor Mitt Romney said during a town-hall meeting Friday in Manchester, N.H. “On almost every dimension, what he did did not help the economy get out of the slide it was in, but instead he extended the downturn and made it deeper.”

Romney added: “He can’t keep blaming George Bush. This is now his economy.”

Former Minnesota governor Tim Pawlenty said in a statement, “Today’s underwhelming job numbers report demonstrates President Obama’s failure to address the tough challenges we face as a nation.”

GOP strategists pointed to recent polls showing Obama’s weakness on the issue. A CNN poll published late last month found that 58 percent of Americans — including nearly two-thirds of independents — disapproved of the president’s job on the economy. Independents are viewed by Obama aides as a group crucial to his reelection bid.

Still, other polling numbers show that Obama continues to enjoy some advantages, including personal popularity. Fifty-two percent of Americans approve of his overall job performance, and nearly two-thirds said he “cares about people like me.”

“The main worry for Republicans at this point is nominating a strong and credible potential president,” said Whit Ayers, a GOP pollster. “I think we will, but obviously if you want to replace a current president, you have to nominate a credible alternative.”

Staff writers Philip Rucker, Perry Bacon Jr. and Dan Balz contributed to this report. Balz reported from New Hampshire.