This vote was on an attempt to bring debate on a measure to a close (known as a "cloture motion" in the Senate). The bill in question is an ambitious undertaking that would strengthen security at the U.S.-Mexico border, as well as significantly overhauling U.S. immigration policies.

If the Senate votes to "invoke cloture" – or bring debate to a close – then lawmakers must either hold a vote on the legislation, amendment or motion in question, or move on to other business. This type of motion is most often called on contentious legislation where the leadership is concerned that consideration could be held up indefinitely by a handful of unhappy politicians. Debate on the measure itself was contentious and stretched over several days. The Senate's Democratic leadership filed the cloture motion to try to bring these days of debate to an end, as well as to ensure they could defeat any potential filibuster. Invoking cloture, and cutting off a filibuster, requires the same amount of votes (60, a large margin in the Senate).

"Mr. President, this has been a long journey to try and bring our broken immigration system and our broken borders to the place where this Senate can take action. Today's action is going to be absolutely key to whether we will be able to continue and finalize this legislation at the end of the week. So today's vote is a critical vote, key vote, perhaps the most important vote we have had here on this issue over the period of the last 3 years," said Ted Kennedy, D-Mass. and one of the main sponsors of the bill.

The bill 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, in an effort to encourage the estimated 12 million illegal immigrants living in the United States to legitimize their residency.

The heart of the debate over how to change America's immigration system centers around whether the estimated 12 million illegal immigrants currently living in America and contributing to its economy should be given a chance at citizenship (often called "amnesty"), or instead deported for being here illegally. This question has engendered significant strife and has not been easy to negotiate, particularly since it would mean breaking up some families. (Many illegal immigrants have lived in the U.S. for decades and have given birth to children who by law are considered American citizens; in many cases, the parents would have to be deported, but their children would not.) Though these questions sometimes split down regional rather than partisan lines, Democrats generally favor some sort of partial amnesty. Republicans generally oppose any such attempts, believing that amnesty essentially rewards immigrants that have flouted U.S. immigration laws, even as others have patiently waited their turn at legal citizenship.

By a vote of 46-53, the motion to end debate on the measure and proceed to a final vote failed. In essence, this vote was the test vote for the bill's ultimate success or failure. Because it failed, Senate Majority Leader Harry Reid, D-Nev., declared the issue dead for the year. This is because if the Senate cannot muster enough votes to invoke cloture (60), then it also cannot muster enough support to cut off any planned filibusters, which also requires 60 votes. Most Democrats who voted supported ending debate on the measure, though 15 voted against the cloture motion. And most Republicans voted against ending debate on the measure, though 12 voted for the cloture motion. All of the most progressive members of the Senate voted to end debate.. Thus, cloture was not invoked. Because Democrats could not muster enough support to end debate and move to a final vote on the bill, Reid declared the issue dead for the year and pulled it from the Senate's schedule.