WASHINGTON — The United States can no longer afford to train foreign scientists and engineers and then send them back home to work for the nation’s competitors, say lawmakers who are expected to vote Thursday on whether to grant thousands of visas to highly skilled foreign-born graduates.

As an example, more than 20 percent of graduate students at Duke University in North Carolina are from other countries. Students from India, China and South Korea do ground-breaking research on cancer research, electromagnetics and space physics, among other fields.

“And the minute they graduate, we send them home,” said Christopher Simmons, associate vice president of federal relations for Duke University. “For as long as I’ve been in higher ed, there has always been a conversation about why do we do this. We’re going against our self-interest as a country.”

Democrats and Republicans appear to agree that there’s a need to retain highly skilled foreign-born graduates. But they’re still arguing over how to do it: Republicans would welcome 50,000 highly trained immigrants by cutting the same number of immigrants who now arrive through a random lottery. Democrats would rather add the extra science-related visas to the country’s overall immigrant intake. And despite the House vote expected this week, that partisan dispute could put off final passage of a visa expansion until beyond the November election.

Supporters of increasing the number of science graduate visas are excited that lawmakers are now willing to address the issue apart from a more comprehensive immigration policy, as Congress has been reluctant to do before.

The House is expected to consider a bill Thursday introduced by House Judiciary Committee Chairman Lamar Smith, R-Texas, that would grant 55,000 visas to foreign graduates in science, math, technology and engineering. The visas, for what is known as STEM graduates, would come from a pool of visas currently maintained by another program that would be eliminated — the diversity visa program. It awards 55,000 green cards a year to immigrants around the world in a lottery.

Democrats, however, are reluctant to agree with a bill that cuts into the diversity visa program. Rep. Zoe Lofgren of California, the ranking Democrat on Judiciary’s immigration subcommittee, this month introduced a bill that would add 50,000 new green cards for STEM graduates without cutting the diversity visa program. Sen. Charles Schumer, D-N.Y., introduced a similar bill.

The diversity program has come under fire from conservative critics who say it fuels irresponsible immigration. Smith contends eliminating the diversity visa program tightens the country’s immigration policy by reallocating green cards to the best foreign graduates.

But critics, such as Rep. Luis V. Gutierrez of Illinois, chairman of the Congressional Hispanic Caucus task force on immigration, want some kind of compromise.

“Republicans are only willing to increase legal immigration for immigrants they want by eliminating legal immigration for immigrants they don’t want,” Gutierrez, a Democrat, said in a statement.

Advocates for creating a path for illegal immigrants worry that STEM legislation ultimately could increase labor shortages in other fields. Ali Noorani, executive director of the Washington-based National Immigration Forum, cited labor shortages in agriculture and argued for a compromise that “doesn’t rob Peter to pay Paul.”

“The issue is that while high-skilled immigration is important, we’re forgetting that we need high-skilled farm workers as much as we need high-skilled engineers,” Noorani said.