Washington (CNN) - Backers of a bipartisan Senate immigration reform plan Thursday defeated an attempt by Republican opponents to alter the border security requirements in the bill, which could have significantly undermined support for the compromise legislation.
By a vote of 57-43, senators rejected the proposal by Sen. Charles Grassley, R-Iowa, to delay the legalization process for millions of undocumented immigrants in the United States until the Department of Homeland Security could certify it had effective control over the Southern border for six months.
Grassley, who as the top Republican on the Judiciary Committee is managing the bill for his party, complained Democrats used a procedural motion to avoid a direct vote on his amendment.
"It appears the majority is afraid to have an open debate and vote on a provision that ensures true border security before legalization," Grassley said. "They claim to be open to improving the bill, but this motion to table shows that they're not really ready to fundamentally change the bill."
Sen. Marco Rubio, R- Florida, a member of the so-called bipartisan gang of eight that negotiated the bill, voted against Grassley's amendment.
"Our country needs to identify and register those who are here illegally as quickly as possible," Rubio said. "We don't want the problem to get worse. If we wait three or four years to register them, instead of 10 million you're going to be dealing with 14 million."
Sen. Lisa Murkowski of Alaska was the only Republican not part of the "gang" to vote against Grassley, a development that buoyed Democratic leaders' hopes the bill ultimately will pass with more than a handful of GOP votes.
Also Thursday, there were positive signs coming from gang of eight members surrounding fresh negotiations to bolster border security in the bill and win the support of conservative critics.
"We're working on a border security measure that can be supported both by people who support a path to citizenship and people who want further border security, and I'm optimistic we can find something," said Sen. Chuck Schumer, D-New York.
Negotiators are looking for "effective metrics that give us confidence that we're not just spending money, it's actually achieving something," said Sen. Jeff Flake, R-Arizona, who indicated he feels "better today than yesterday" about the course of the talks.