WASHINGTON - President Barack Obama on Thursday signed into law a package of tax breaks and spending designed to give the nation a jobs boost by encouraging the private sector to start hiring again.

It's the first of several such measures Democrats have promised this election year to address the public's top worry: jobs. The measure includes about $18 billion in tax breaks and pumps $20 billion into highway and transit programs.

At a ceremony in the sunny White House Rose Garden, Obama said the bill is necessary "but by no means enough."

"There is a lot more we need to do to spur hiring in the private sector and bring about full economic recovery," he said.

There is plenty of skepticism that the new law will do much to foster hiring. Optimistic estimates are that the tax break could generate perhaps 250,000 jobs through the end of the year; some 8.4 million jobs have been lost since the start of the recession.

Small businesses in particular will benefit, the president said.

"Many of them are on the fence right now about whether to bring on that extra worker or two, or whether to hire anyone at all," Obama said. "This jobs bill should help make their decision that much easier."

Under the new law, businesses that hire anyone unemployed for at least 60 days would be exempt from paying the 6.2 percent Social Security payroll tax through December. Employers also would get an additional $1,000 credit if new workers remain on the job a full year. Taxpayers will have to reimburse Social Security for the lost revenue.

The new law also extends a tax break for small businesses buying new equipment and modestly expands an initiative that helps state and local government pay for transportation and infrastructure projects.

It is paid for over the coming decade in part by cracking down on offshore tax havens, though it would add $13 billion to the debt in the coming three years.

In a show of bipartisanship toward the election-year goal of job creation, the Senate passed the bill Wednesday with 11 Republicans among the 68 senators who voted to send it to the president.