WASHINGTON — In an early test of support for a comprehensive immigration bill, the Senate on Thursday voted down a Republican attempt to require the U.S.-Mexico border to be under control for six months before immigrants here illegally could take the first steps toward citizenship.

Supporters of the bill said the GOP amendment would have delayed for years the path to citizenship at the center of the legislation.The amendment’s author, Sen. Charles Grassley, R-Iowa, said it was needed to ensure the bill made good on its promises of ushering in true border security.

The measure failed 57 to 43, suggesting that bill supporters have work to do to lock down the 60 votes that will likely be needed to overcome GOP stalling tactics and secure final passage of the bill several weeks from now.

Grassley’s was the first amendment voted on regarding the White House-backed legislation to remake U.S. immigration system, boost border security and workplace enforcement, and create a path to citizenship for some 11 million people now here illegally.

The bill allows people illegally in the United States to obtain a new “registered provisional immigrant” status six months after enactment, as long as they meet certain criteria and the secretary of Homeland Security has developed plans to secure the border and erect new border fencing where needed.

Critics say that developing a plan does not actually strengthen border security, and Grassley said his amendment was designed to address that flaw.

His amendment said that no one could obtain registered provisional immigrant status until the Homeland Security secretary has certified to Congress that the border has been under “effective control” for six months. Effective control is defined as surveillance of the entire border and catching or turning back 90 percent of people attempting to cross the border.

“My amendment ensures the border is secured before one person gets legal status,” said Grassley. “It’s a common sense approach. Border security first, like promised, legalize next.”

Sen. Chuck Schumer, D-N.Y., one of the bill’s authors, said that achieving border control could take years, during which time more immigrants would cross into the U.S. illegally and there would be no solution for the millions already here.

“This clearly would undo the entire theme and structure of the immigration bill that has such bipartisan support,” Schumer said. “And we may never get to real immigration reform that is needed, so desperately needed by the country.”

Despite the failure of Grassley’s amendments the bill’s supporters are looking for some border security measures they can accept. Given GOP suspicion of the border security provisions in the bill, some changes will be needed to ensure the bill has the support necessary to pass the Democratic-controlled Senate and ultimately the GOP-controlled House.

But Schumer and others insist that they won’t agree to changes that fundamentally alter the carefully crafted compromises at the center of the bill or delay the path to citizenship, already designed to take at least 13 years, for those who attempt it.

Public polling shows general support both for tougher border security and for allowing those living in the United States to gain citizenship after meeting certain legal, financial and other conditions. On an issue as contentious as immigration, that made the intersection of the two a fertile ground for dispute.

Senate Majority Leader Harry Reid, D-Nev., has said he wants a final vote on the measure before July 4.

Across the Capitol, House Speaker John Boehner, R-Ohio, says he hopes immigration legislation can move through committee by then, and be on the floor sometime in July.

While the obstacles to a final agreement are daunting, the Senate bill has support from both business and organized labor, two groups that are usually on opposite sides of most issues.

Additionally, senior Republicans have made it clear they envision the legislation as a way for the party to show a friendlier face to Hispanic voters, who overwhelmingly supported President Barack Obama last fall.