President Trump threw his support Wednesday behind a Senate bill that would cut legal immigration in half and impose a merit-based system, giving preference to English-speaking immigrants who demonstrate job skills and curtailing the traditional pipeline that rewarded extended family ties.

Meeting at the White House with Republican Sens. David Perdue of Georgia and Tom Cotton of Arkansas, the bill’s sponsors, the president said the legislation would be the biggest change to immigration policy in 50 years. His aides signaled that they expect it to be a major part of the national debate heading into midterm elections next year.

Democrats vowed to resist the changes, and immigrant rights groups said Mr. Trump was catering to “white nationalists” with the proposal, which would slash legal immigration over the next decade from about 1.1 million green cards a year to 500,000.

The bill would also prevent immigrants from accessing welfare and would replace the employment visa system, which relies on businesses to pick immigrants, with a skills-based system that gives the government a bigger role in selecting applicants based on their individual merits.

“This legislation demonstrates our compassion for struggling American families who deserve an immigration system that puts their needs first and that puts America first,” said Mr. Trump, adding that it would “restore the sacred bonds of trust between America and its citizens.”

Mr. Cotton said the current immigration system is “an obsolete disaster,” in which only 1 immigrant in 15 comes to the U.S. because of job skills.

“I think it’s a symbol that we’re not committed to working-class Americans,” Mr. Cotton said.

Congress has repeatedly toyed with cutting legal immigration and with enacting a points-based system to pick immigrants — most recently in 2007, in an immigration bill President George W. Bush and leaders of both parties tried to push through the Senate.

The bill failed, but most Senate Democrats voted for it.

Now, with Mr. Trump’s stamp on the policy, Democrats are adamantly opposed. House Minority Leader Nancy Pelosi, California Democrat, called the proposal part of a “hateful, senseless anti-immigrant agenda.”

Immigrant rights activists were, if anything, even harsher.

“Let’s call it as we see it: This is a white nationalist agenda masquerading as a bill about skill levels,” said Frank Sharry, executive director of America’s Voice.

Business groups also announced opposition, saying the country needs more immigrant labor in order to keep up in a global economy.

Compete America, a pro-immigration lobby, said the bill amounted to the government “telling American employers who they should hire.”

The system proposed by Mr. Cotton and Mr. Perdue would reward education, English-language ability, high-paying job offers, achievements and entrepreneurial initiative.

The White House said it would be similar to the merit-based immigration systems used by Canada and Australia.

The measure prioritizes immediate family members of U.S. residents, including spouses and minor children, but would end preferences for extended family members and adult children.

Legal permanent immigrants are issued what is known as a green card, proving their status in the U.S. Green card holders can usually apply for citizenship after five years, can sponsor relatives for immigration and can access welfare benefits at some point.

A majority of green cards are issued to people already living in the U.S. on temporary visas — though that number has been dropping over the past decade.

Americans are divided on the right level of legal immigration. About 40 percent want to see the numbers cut, while another 40 percent want them to stay the same. The remaining 20 percent want increases.

The U.S. system is widely seen as broken, however, with immigrants — both legal and illegal — often having more say in the matter than the government.

Extended families, business relationships and even a random lottery are used to award permanent visas.

It wasn’t always that way.

From 1924 to 1965, the country imposed strict immigration limits, including nationality quotas, but granted exemptions to close family members and applicants with high skills or who were brought in to work in agriculture.

Congress decided to liberalize the policy in 1965, creating the framework for the modern system that focuses heavily on extended family ties. It also abolished national quotas.

The result was a system where about two-thirds of the green cards issued each year are for immediate relatives of U.S. citizens, and about 15 percent go to refugees and asylees. Another 15 percent go to employment-based applicants.

The remainder go to winners of the diversity visa lottery, established in 1990, which doles out green cards based on chance.

The goal was to give potential immigrants who don’t have family ties or job prospects a shot at making it to the U.S.

The bill would nix the lottery, with the Trump administration saying it “serves questionable economic and humanitarian interests.”

The bill would also limit permanent resident status for refugees to 50,000 a year, which the White House says is in line with the average over the past 13 years.

Trump senior adviser Stephen Miller suggested that the White House intends to make the proposal a campaign issue next year in congressional midterm elections.

“Ultimately, members of Congress will have a choice to make,” Mr. Miller said. “They can either vote with the interests of U.S. citizens and U.S. workers, or they can vote against their interests, and whatever happens as a result of that, I think, would be somewhat predictable.”

Mr. Miller said polls consistently show that Americans favor immigration policy that requires new arrivals to speak English, prevents immigrants from displacing existing workers, bars immigrants from receiving welfare, requires them to have skills and reduces overall net migration.

“I do think that voters across the country are going to demand these kinds of changes,” he said. “The effect it has on their lives and their communities is overwhelmingly positive.”