Sen. Marco Rubio (R-Fla.) wants to strengthen requirements in a sweeping immigration bill that mandate that illegal immigrants learn English before earning permanent U.S. residency.

Under the current bill, immigrants would have to earn English proficiency or show they are enrolled in a language course. Rubio, a member of the bipartisan group that developed the legislation, plans to offer an amendment that would eliminate the second provision and require that undocumented immigrants be able to read, write and speak English before earning a green card.

The Senate bill allows undocumented immigrants to apply for a green card after 10 years and then apply for citizenship three years later.

“On the day we announced the principles that would shape the immigration bill, we made it clear that English proficiency would now be required for permanent residency for the first time in American history,” Rubio said in a statement. “This amendment ensures that will be the case."

Rubio is considered a key member of the bipartisan group because of his appeal to conservatives, but he has said recently that the legislation will not earn enough Republican support without changes to strengthen border security.

That has left Democrats and immigration advocates fearful that opponents will lard the bill with too many roadblocks for immigrants to earn citizenship.

“Since the bill was introduced two months ago, the open and transparent process it has undergone has elicited constructive criticisms to improve it,” Rubio said. “This is one of the bill’s shortcomings that came to light, which we can now fix.”

The Senate is expected to vote Tuesday on a motion to proceed with the immigration bill, and members on both sides of the aisle are developing a lengthy list of amendments. Senate leaders have said they hope to hold a vote on the bill before July 4.