Republican Sen. Marco Rubio of Florida announced Tuesday he will introduce an amendment to the immigration reform bill that would require all immigrants to prove they were proficient in English before they could receive permanent legal immigration status.

Rubio’s amendment would significantly change the sweeping bill and would make it harder for millions of immigrants to get on the path to citizenship. It would remove language from the bill that would have also allowed immigrants to gain green cards if they enrolled in a government-approved English course, a provision that Rubio called a “loophole.”

“This is one of the bill’s shortcomings that came to light, which we can now fix,” Rubio said in a statement.

A tea party favorite of Cuban descent, Rubio has been key in drawing conservative support to the bill, which he helped draft as part of the bipartisan Gang of Eight. But since then, Rubio has said the bill must be reworked on border security and other issues before he will vote for it.

At least one anti-immigration reform group is running ads targeting senators for the provision, criticizing the bill for having “no requirement” that immigrants learn English before gaining legalization.