WASHINGTON » Landmark immigration legislation passed by the Senate would remake America’s workforce from the highest rungs to the lowest and bring many more immigrants into the economy, from elite technology companies to restaurant kitchens and rural fields.

In place of the unauthorized workers now commonly found laboring in lower-skilled jobs in the agriculture or service industries, many of these workers would be legal, some of them permanent-resident green card holders or even citizens.

Illegal immigration across the border with Mexico would slow, but legal immigration would increase markedly.

That’s the portrait that emerges from recent analyses of the far-reaching bill passed last month by the Senate with the backing of the White House. Although the bill aims to secure the borders, track people overstaying their visas and deny employers the ability to hire workers here illegally, it by no means seeks to choke off immigration. Indeed, the U.S. population over the next two decades would be likely to increase by 15 million people above the probable level if no changes were made to immigration laws, according to the Congressional Budget Office.

Even after decades of growth in the U.S. foreign-born population, the added increase could be felt in ways large and small around the country, from big cities that would absorb even more diversity to small towns that may still be adjusting to current immigrant arrivals.

"That is baked into the basic premise of the bill," said Doris Meissner, a senior fellow at the Migration Policy Institute, "which is that you need to provide legal avenues for people to come to the country both in longer-term temporary and in permanent visa categories in order to meet the needs of the future and avert the incentives for illegal immigration."

The level of immigration under the legislation has been a political issue in the debate and will probably continue to be disputed in the weeks ahead as the House’s GOP majority wrestles with how to respond to the Senate bill. It’s a complex question because the Senate version expands various temporary and permanent visa categories, shuts down others and creates new ones. Some visa programs are capped and some aren’t, and some would expand or contract in response to demand.

Opponents led by Sen. Jeff Sessions, R-Ala., have forecast dramatic increases in immigration under the bill, with Sessions warning that 57 million new permanent and temporary residents and newly legalized immigrants would flood the U.S. within the decade and rob Americans of jobs. On the other side, supporters including Sen. Marco Rubio, R-Fla., have downplayed the impact of the bill. In response to Sessions, Rubio’s office argues that the Senate bill "does not significantly increase long-term, annual migration to the United States."

Under current law, around 1 million people get green cards granting permanent U.S. residence each year. That would rise to between 1.5 million to 1.7 million annually under the Senate bill within about five years of enactment, the Migration Policy Institute estimates.

But those figures don’t count people coming to the U.S. under temporary worker visas, and their numbers could rise by hundreds of thousands a year under the Senate bill, according to the Institute. This includes more than twice as many visas for high-skilled workers, a new visa category for lower-skilled workers that could go up to 220,000 a year, and more visas for agricultural workers. There are also tens of thousands of new work visas set aside for people from Ireland, South Korea, African and Caribbean countries, and elsewhere that got special deals in the bill. Some of these workers would be able to transition to permanent status and eventually citizenship.

On the other side, the flow of illegal immigration into the country would decrease by one-third or one-half compared with current law, the Congressional Budget Office says. Illegal immigration has already decreased since 2000 due to a combination of factors, including the economic downturn and greater security measures in the wake of the Sept. 11, 2001, terror attacks. Although up-to-date annual figures on illegal immigration are hard to come by, one recent study published in the International Migration Review said that close to 400,000 people entered illegally in 2009, either by crossing the border unlawfully or overstaying temporary visas. An author of the study, Robert Warren, said that figure probably has not changed dramatically in the years since.

The Senate bill offers a 13-year path to citizenship for the estimated 11 million immigrants already here illegally, the most contentious element of the legislation since many House conservatives oppose granting citizenship to people who broke U.S. laws to be here. But that aspect of the legislation has little impact on the overall population size since the people involved are already in the country even if they end up transitioning to legal status. The Congressional Budget Office estimates that some 8 million of them would do just that.

Beyond the changes in numbers, the immigration bill would shift the emphasis of U.S. immigration policy away from family ties and put more weight on employment prospects, education and relative youth. It also would raise ceilings on how many immigrants could come from any one country. And there would be impacts as yet unforeseen as the policies unspool into an uncertain future and economic conditions in other countries affect how many foreigners dream of calling the U.S. home.

"There’s not going to be a dramatic change that we will see overnight, but longer-term changes," said Audrey Singer, a senior fellow at the Brookings Institution. "We’ve got this idea of the policy and then when we put it into practice inevitably there are unintended consequences or unintended trends that develop."