Updated 4:45 p.m. |

All 15 senators voting to filibuster debate were Republicans: John Barrasso of Wyoming, John Boozman of Arkansas, Michael D. Crapo of Idaho, Ted Cruz of Texas, Michael B. Enzi of Wyoming, Charles E. Grassley of Iowa, James M. Inhofe of Oklahoma, Mark S. Kirk of Illinois, Mike Lee of Utah, Jim Risch of Idaho, Pat Roberts of Kansas, Tim Scott of South Carolina, Jeff Sessions of Alabama, Richard C. Shelby of Alabama and David Vitter of Louisiana.

The Senate later voted 84-15 on the motion to proceed to the bill itself, with Grassley switching his vote from “no” on cloture to “yes” on proceeding to the bill and Sen. Thad Cochran, R-Miss., switching from “yes” on cloture to “no.”

It was the first test of what is sure to be many for the “gang of eight” framework that passed out of committee last month. Senate Majority Leader Harry Reid, D-Nev., has said he expects several weeks of deliberation before final votes and the July Fourth recess.

But the road to final passage is not clear, even if the bill’s backers are confident about their chances. Republicans are pushing for stronger border control provisions while senators from both sides of the aisle mull introducing amendments that are either non-germane to the bill or could disrupt the delicate political coalition championing the legislation — from expanding the bill to include better protections for gay immigrants to gun control provisions to a series of amendments that would weaken the core of the bill from Republican senators who have little intention to vote for a final package.

Meanwhile, Sen. Orrin G. Hatch, R-Utah, is still waiting for all four of his amendments to be adopted on the floor so he can back the bill he voted to approve out of committee.

Minority Leader Mitch McConnell of Kentucky was among the Republicans voting to start debate but demanding “major changes” to the bill before he will support it, even as President Barack Obama held an event at the White House urging Congress to send him the immigration overhaul by the end of the summer.

Let the race to 60 votes — on final passage — begin.