WASHINGTON – Proponents of immigration reform are aiming to attract 70 votes when the issue hits the floor, but that hope may be dimming as some prominent supporters appear to be backtracking on their earlier commitments.

The chief defector could be Sen. Marco Rubio (R-Fla.), a member of the Gang of Eight who pieced together the legislation. Rubio, a potential candidate for the 2016 GOP presidential nomination, recently indicated he’s not satisfied with the bill’s border security provisions and won’t vote for it unless significant changes are made during the amendment process.

“And so there’s a handful of Democrats, and a sizeable number of Republicans that are saying to us we’re prepared to do immigration reform, but we have to make sure there isn’t another wave of illegal immigration,” Rubio said June 4 on Hugh Hewitt’s radio program. “So for those who want immigration reform, the task is very simple. Let’s strengthen the border security parts of this bill so that they’re stronger, so that they don’t give overwhelming discretion to the Department of Homeland Security, and I think if we can do that, then you’re going to be able to get something done. But if you can’t, it’s not going to happen.”

Rubio’s apparent retreat is somewhat surprising since the security provisions contained in the legislation were actually strengthened before it emerged from the Senate Judiciary Committee last month. Rubio has served as both the face and the voice pushing the bipartisan immigration reform proposal.

In a letter to Arizona Attorney General Tom Horne, who also questioned the bill’s security provisions, the state’s two Republican senators, John McCain and Jeff Flake, both members of the Gang of Eight, assured that necessary steps have been taken to bolster the border.

Under the bill, the secretary of the Department of Homeland Security would be required to provide Congress with a plan to maintain persistent surveillance along the border that would stop more than 90 percent of illegal entries. The department furthermore would be required to report to Congress on how effectively it is implementing its plan, any impediments to fulfilling the plan, and what actions it will take to address such impediments.

“We agree that this administration has left much to be desired with regards to securing the border,” the two lawmakers said. “This is the reason why, rather than relying on the subjective interpretation of any elected bureaucrat, we developed an approach to border security that would be based on objective metrics to provide empirical data to determine the level of security along the border, a strategic plan that will allow Congress to hold DHS accountable to doing so, and an unprecedented amount of resources dedicated to that very effort.”

Regardless, Rubio’s withdrawal, if it comes to that, can’t be good for the bill’s future. His earlier support could provide cover for other lawmakers who are on the fence to oppose the measure.

Sen. Rob Portman (R-Ohio), who maintains he supports immigration reform and whose vote has been targeted by the bill’s proponents, also claims the measure’s enforcement provisions are too weak. He will oppose it on the floor unless changes are made.

Speaking to Newsmax, Portman asserted that the last time Congress adopted an omnibus immigration reform measure – the Simpson-Mazzoli Act of 1986 – the nation “did not enforce the law either at the border or internally.”

Portman also is looking at changes to strengthen E-Verify, the computerized system employed to determine if a potential worker is in the country legally. Employers are not required to access E-Verify under the bill headed to the floor. The lawmaker told Newsmax that a robust E-Verify system is essential.

“We need to be sure that we’re not moving ahead with the legal status of some kind for people who are here without putting enforcement in place, and that’s something I’ll be working on on the floor of the Senate,” Portman told Newsmax.

The Border Security, Economic Opportunity and Immigration Modernization Act of 2013, as it currently stands, creates a pathway to citizenship for the estimated 11 million immigrants who entered or remained in the country without proper documentation.