Four Republican senators are stepping up their opposition to a comprehensive immigration bill, arguing in a new letter to colleagues that the legislation fails to secure the U.S. border with Mexico and unfairly rewards illegal immigrants with a chance at citizenship.

Senate Judiciary Committee member Sen. Jeff Sessions, R-Ala. (AP Photo/J. Scott Applewhite) Senate Judiciary Committee member Sen. Jeff Sessions, R-Ala. (AP Photo/J. Scott Applewhite)

Sens. Ted Cruz (Texas), Chuck Grassley (Iowa), Jeff Sessions (Ala.) and Mike Lee (Utah) detail nine major concerns they have with the sweeping proposal that was approved by Senate Judiciary Committee two weeks ago.

The four lawmakers, all members of that committee, voted against the bill, and they said the committee failed to improve the proposal because it rejected amendments offering "real solutions."

"The bill's already serious flaws were exacerbated by the adoption of several amendments that significantly weaken current law, hamstring law enforcement, and further complicate our legal immigration system," they wrote.

The letter comes as the full Senate is expected to begin debating the immigration bill next week, with a vote potentially coming before the July Fourth holiday. The legislative proposal, developed by a bipartisan group composed of four Democrats and four Republicans, represents the most ambitious overhaul of immigration law in nearly three decades.

Opponents of the bill have focused mostly on border security and the 13-year path to citizenship for the nation's 11 million immigrants who entered the country illegally. That debate is expected to intensify during the Senate floor deliberations, and Sen. Marco Rubio (R-Fla.), a key member of the bipartisan group that developed the bill, has said the legislation will need to be amended to win enough support from Republicans to ensure passage.

Rubio has focused on border control as an area where he expects changes. But Democrats in the bipartisan group are wary about how far to the right the legislation can move without losing support from liberals who fear that the path to citizenship could be delayed or eliminated with the addition of tighter border control requirements.

In their letter, Cruz, Grassley, Sessions and Lee highlighted several amendments they offered to beef up border security and limit the legal status for some of the illegal immigrants. One amendment, from Grassley, would have required the Department of Homeland Security to certify to Congress that it has maintained effective control over the entire southern border for six months before undocumented immigrants gain legal status.

The committee "voted down every attempt to mandate meaningful control of our borders," the senators wrote. "Americans expect their government to end the lawlessness, not surrender to it."