WASHINGTON - Key Senate Republicans are working to develop a compromise on border security that would satisfy GOP demands for stronger enforcement language in a far-reaching immigration bill without costing Democratic support, lawmakers and aides said Thursday.

To win over skeptical Republicans, senators are considering mandating specific requirements for equipment and other tools along the U.S.-Mexico border, instead of just leaving it up to the Obama administration, said Sen. John McCain, R-Ariz., an author of the bill.

"That may be a way to assuage the concern of some of our friends that are concerned about border security," McCain said. Sen. Marco Rubio, R-Fla., another author, has discussed the same approach.

The talks were underway behind the scenes at the Capitol Thursday even as the Senate voted 53 to 47 to defeat an amendment by Sen. Charles Grassley, R-Iowa, that would have required a controlled border for six months before any immigrant here illegally could take the first steps toward citizenship. It was the first amendment the Senate voted on to the White House-backed immigration legislation.

The outcome suggested that bill supporters have work to do to lock down the 60 votes that will likely be needed to overcome GOP stalling tactics and get it passed in the Senate by July Fourth, the timeline set by Senate Majority Leader Harry Reid.

The bill, which would amount to the most significant changes to immigration law in decades, would require all employers to check workers' legal status, allow tens of thousands of new high- and low-skilled workers into the country, and create a 13-year path to citizenship for some 11 million people now here illegally.

The bill also devotes billions to new equipment and personnel along the U.S.-Mexico border, and says the path to citizenship can't go forward until certain border-security requirements are met.

"If we pass the bill as it is, there will be no pressure on this administration or future administrations to secure the border," Grassley said.

Rubio has been saying that stronger language on border security would be needed to ensure passage in the Democratic-controlled Senate and the Republican-controlled House. The question now is how to do that without raising concerns among Democrats that the path to citizenship would be delayed - the reason that Sen. Chuck Schumer, D-N.Y., and others gave for opposing Grassley's amendment.

In the absence of an amendment that answers Republican concerns without alienating Democrats, Republicans have been voicing support for a measure by Sen. John Cornyn, R-Texas, that requires 100 percent surveillance of the border and 90 percent of would-be crossers to be stopped before anyone can get a permanent resident green card. The bill as written includes those same figures as goals but doesn't make the citizenship path contingent on them.