As President Donald Trump wrestles over the details of a DACA deal with Democrats, he gave an assurance to conservatives Friday morning.

“CHAIN MIGRATION cannot be allowed to be part of any legislation on Immigration!” the President tweeted.

Chain migration is the process by which one immigrant is admitted to the country, then he or she sponsors relatives back home to come to the U.S., who in turn could sponsor more relatives. In other words, under current U.S. immigration policy, admitting one immigrant to the country who can sponsor family members can set off a chain reaction that swells immigration numbers.

Trump’s tweet came as he and Democratic leaders are trying to pin down details of an immigration deal that could grant protections for Dreamers, certain people brought to the U.S. illegally as children. Democrats are pushing the President to support the DREAM Act, which would provide a pathway to citizenship for Dreamers.

But some conservatives have warned that this would set off a massive chain migration process. “If you replace DACA with the DREAM Act, that will be 3 million plus,” the Federation for American Immigration Reform tweeted. After chain migration it will be the biggest amnesty in modern history.”

“The problem with any amnesty is that it serves as a magnet for new illegal immigrants, and its recipients could become the next link in chain migration if granted legal status or especially citizenship,” wrote the editorial board of the conservative publication National Review.

Republican senators Tom Cotton of Arkansas and David Perdue of Georgia have introduced their own immigration bill, the RAISE Act, which would cut legal immigration by half and excise adult children and siblings from the chain, only allowing pathways for spouses and minor children of people admitted to the country.

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Write to Tessa Berenson at tessa.berenson@time.com.