This vote occurred on an attempt to bring debate on a bill to a close (known as a "cloture motion" in the Senate). If the Senate votes to "invoke cloture" – or bring debate to a close – then lawmakers must either hold a vote on the legislation or amendment in question, or move on to other business. This type of motion is most often called on contentious bills or amendments where the leadership is concerned that consideration could be held up indefinitely by a handful of unhappy politicians.

The motion was called on a substitute amendment that represented a bipartisan compromise reached between Democrats and Republicans on changing immigration policies. It was offered by Ted Kennedy, D-Mass., to an early version of a bill that would overhaul the immigration system. A "substitute amendment" is one that replaces the entire text of a bill with new text. This approach is generally taken when so much of the original bill is being changed that it's simply easier to replace the entire thing than amend it piecemeal.

The substitute amendment represented a hard-won compromise between competing sides of the immigration debate: those who want to give the nearly 12 million illegal immigrants a chance to legitimize their residency either temporarily or permanently, and those who want to tighten security along America's border with Mexico. To that end, the bill would establish several new visas that allow immigrants to work temporarily or put them onto a path to citizenship. But for these visas to be activated, the government would be required to first certify that several other initiatives intended to strengthen America's border included in the underlying bill have been implemented.

After nearly two weeks of plowing through amendments, most of them offered unsuccessfully by Republicans, Senate Majority Leader Harry Reid, D-Nev., called a cloture vote to bring debate on the measure to a close and move the Senate toward a final vote on the bill.

Republicans complained bitterly that Democrats had not allowed them enough time to offer all the amendments they wanted, while Democrats answered that they had days in which to offer amendments and that it was time to wrap things up.

"I said earlier this week that Republicans would not allow themselves to be stuffed on this bill. The Senate isn't a factory. We don't push things down the line," said Senate Minority Leader Mitch McConnell, R-Ky.

Senate Majority Leader Harry Reid, D-Nev., answered that Republicans are doing little more than engaging in delaying tactics, and that even after cloture is invoked Republicans will have an extra 30 hours in which to offer and debate amendments.

"We have spent a lot of time on the Republicans delaying what the American people want us to do, and that is legislate. In spite of that, we were able to move on and do some significant legislating, as we are going to continue to do," Reid said.

By a vote of 33-63, the motion to end debate was defeated. Every Republican voted against the motion. Most Democrats voted for the motion, though 15 voted against it. Independents were split, with Joe Lieberman of Connecticut voting for the motion to end debate, and progressive Bernie Sanders of Vermont voting against it. Progressive Barbara Boxer, D-Calif., also voted against ending debate. Both Boxer and Sanders spoke vigorously against portions of the underlying bill that would allow more temporary workers, believing it would harm the American workforce. And earlier during debate on the bill, both Boxer and Sanders offered amendments seeking to strike or water down these provisions (both failed). Thus, cloture was not invoked and debate on the immigration bill continued.