President Donald Trump on Friday said he drew the line at allowing so-called chain migration in any potential immigration package he might negotiate with help from Democrats.

“CHAIN MIGRATION cannot be allowed to be part of any legislation on Immigration!” the president wrote on Twitter.

Chain migration refers to a tactic by which lawful permanent US residents sponsor members of their family, particularly nonnuclear ones, who then sponsor several more family members.

Critics of chain migration, such as the Federation for American Immigration Reform, say it is a significant factor in bringing large numbers of low-skilled immigrants to the US. In a March study, FAIR wrote that the “problem with chain migration is that while the United States retains the ability to say ‘yes’ or ‘no’ to individual immigrants, the pool of immigrants is self-selecting.”

Rep. Jody Hice, a Georgia Republican, introduced a bill to curb chain migration earlier this year called the Nuclear Family Priority Act, which he said in a February statement “would take crucial steps to reform an antiquated immigration selection process and prioritize keeping nuclear families intact.”

Trump’s demand that there be no chain migration in any immigration bills comes on the heels of negotiations with lawmakers over what to do about beneficiaries of the Deferred Action for Childhood Arrivals program, which protects people who are in the US illegally after coming to America as a minor.

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Democrats, however, are unlikely to get on board with that demand. On Thursday, House Minority Leader Nancy Pelosi said a pathway to citizenship would be guaranteed in a bill to protect DACA recipients, which would allow them to sponsor relatives upon becoming a permanent US resident.