President Trump's embrace of a Senate proposal to sharply restrict legal immigration to the United States is the latest iteration of a bitter political debate going back to the 1980s, when Congress first attempted to clamp down on illegal entries.

The proposal then was to legalize an estimated three million illegal immigrants in exchange for tighter border controls. The bill was signed into law but, despite the promises, tighter border controls never happened and by the late 1990s, the number of undocumented immigrants in the country had quadrupled.

That is the context for Trump endorsement of legislation by Republican U.S. Sens. Tom Cotton of Arkansas and David Perdue of Georgia that would reduce legal immigration from about one million a year to half that in 10 years, mostly by eliminating preferences for relatives of legal U.S. residents. Trump's base of blue-collar voters enthusiastically approves of his plan to crack down on illegal immigration and that arguably put Trump over the top in the election.

The calculation in the White House is that a similar approach to legal immigration will get the same backing from voters who believe that foreign workers suppress wages and take jobs from native-born Americans.

Whether any of this becomes law, given the swirling political cross currents on Capitol Hill and the bet by some Republicans that it's better to go it alone in advance of next year's elections than throw in their lot with Trump, is an open question.

But here are a few themes that likely will emerge as the White House pushes the plan.

• Support for the plan in the Senate is weak. Many Republicans, in the thrall of the U.S. Chamber of Commerce, are advocates for permissive immigration rules, arguing that industries such as agriculture, tourism and hotels will suffer extreme labor shortages absent a steady inflow of foreigners. Liberals and immigration activists will join forces with them in pushing back.

• Each side can point to academic studies to bolster their arguments. A study at the University of Pennsylvania's Wharton School last year claims that the effects of immigration are largely positive, and that immigrant workers over the long term do not take jobs away from native-born Americans. In regions with large numbers of unskilled foreign workers, though, such immigrants tend to drain public finances, largely through their impact on public schools, the study said.

• A 2013 study by the Center of Immigration Studies, an advocacy group that pushes for immigration restrictions, concluded that immigrants consume more in public services than they pay in taxes. While an influx of immigrants causes the economy to grow, virtually all of that growth is consumed by the immigrants themselves, or by upper income native-born Americans and Americans with substantial assets, like factory owners.

• Because, for the moment at least, the measure appears unlikely to pass, the chief value of the proposal to the White House may be as a wedge issue in next year's congressional elections. Democrats had hoped their recent unveiling of a new economic plan would rebuild ties with blue-collar workers who flocked to Trump last year. Trump seems to believe that maneuvering Democrats into a spot where they have to defend immigration will keep them back off balance.