By MOTOKO RICH

The United States economy gained momentum in January, adding 243,000 jobs, the second straight month of better-than-expected gains.

The unemployment rate fell to 8.3 percent, giving a cause for optimism as the economy shapes up as the central issue in the presidential election. The Labor Department’s monthly snapshot of the jobs market uses a different survey, of households rather than employers, to calculate the unemployment rate.

Measured by both the unemployment rate and the number of jobless — which fell to 12,758,000 — it was the strongest signal yet that an economic recovery was spreading to the jobs market. The last time the figures were as good was February 2009, President Obama’s first full month in office.

The report sent stock-index futures up sharply on Wall Street before the opening bell.

The department also revised its estimate for new jobs in December slightly, to 203,000. It had reported a net gain of 200,000 jobs in December.

The private sector remained the engine of new job gains. While federal agencies and local governments continued to lay off workers, private-sector employers added 257,000 jobs in January. The industries with the biggest gains were manufacturing, professional and business services, and leisure and hospitality.

The promising jobs numbers came as various economic indicators have painted an ambivalent picture of the recovery’s strength.

Layoffs appear to be slowing as fewer people are filing for unemployment claims, and factory orders have picked up.

But while sales of existing homes have started to rise, home prices continue to fall. Consumer spending is still restrained, and could come under further pressure with oil prices edging higher over the last four months and as consumers revert to building up savings.

“It’s a steady grind in the right direction, but it’s a grind,” said Omair Sharif, United States economist at the Royal Bank of Scotland. “You’re going to have a situation where you take a couple steps forward and a step backward.”

Although the number of unemployed people is finally starting to fall, the number is still about equal to the entire population of Pennsylvania, and long-term unemployment is one of the most crushing legacies of this recent recession. According to an analysis of December’s Labor Department numbers released earlier this week by the Pew Fiscal Analysis Initiative, nearly a third of the jobless have been unemployed for a year or more. In January, the Labor Department reported that 5.5 million people had been out of work for six months or more.

Underemployment is another stubborn problem. The number of people working part time because they could not find full-time work was 8.2 million in January. Including that group and those who have stopped looking for work altogether, the broader measure of unemployment was 15.1 percent.

“You have an interesting situation where you have some permanent part-time workers,” said John Silvia, chief economist at Wells Fargo. “These people are in jobs and the jobs are not likely to become full-time.” He added: “That’s just a new flavor of the labor market.”