Having voted to bring the immigration bill to final action on June 28, Sen. Sam Brownback (R-Kan.) watched his colleagues vote to kill the measure. Ultimately, he gave in and joined them, switching his vote from “yea” to “nay.”

It was an unusual public reversal, but unusual only for being public.

Members of the Senate reverse themselves during votes hundreds of times a year, simply by gaining the attention of the recording clerk and asking that their vote be changed.

Some of the most prominent Senators are also among the most common vote switchers, including presidential candidates Joseph Biden (D-Del.) and John McCain (R-Ariz.), and Majority Leader Harry Reid (D-Nev.), who over the past five years has reversed his vote more frequently than anyone else in the Senate and more frequently than others who have held his leadership positions.

Most of these reversals have no impact on the outcome of the vote and appear to be simply a matter of confusion over which measure is on the floor at the moment, or whether the Senate is voting to pass a bill or to table it.

But some reversals are pivotal, and others indicate a clear decision by Senators to change position once the outcome of a vote has been determined.

For the most part, when a Senator switches his or her position during a vote, it is not a public affair. Only the final vote is recorded in the Congressional Record, and there is no database indicating the real-time progress of roll-call votes.

But the change is reflected on the tally sheets taken by the Senate clerk, who will scratch out the original vote and mark the change. Roll Call reviewed 1,481 tally sheets from the beginning of the 108th Congress through May 15, 2007, and built a database of every vote that was marked as a change on those tallies.

Every Senator who served during that time period switched their vote at least once, and the majority switched their votes between five and 15 times.

But some Senators far outpaced the average.

Biden and Sen. Max Baucus (D-Mont.) switched their votes 31 times, and Indiana Democratic Sen. Evan Bayh reversed himself 30 times. McCain led all Republicans with 28 reversals, followed closely by Sen. Mike Crapo (R-Idaho) with 26.

Reid led all Senators, reversing himself 38 times since January 2003. Reid’s total is undoubtedly tied up with his leadership position because he frequently switches votes to oppose cloture motions and other leadership priorities when it is clear they will not succeed. The Senate Parliamentarian’s office explained that on any legislation, a Senator voting with the prevailing side may offer a motion to reconsider the matter, but the tactic is most commonly used in cloture motions when the vote falls short of cutting off debate. Reid spokesman Jim Manley said he believes most of Reid’s reversals can be accounted for by such strategic considerations.

Thomas Mann, a Congressional scholar at the Brookings Institution, agreed that in Reid’s case, the reversals are “not a matter of indecision or of outside lobbyists getting to him during the vote, but almost entirely a matter of searching for some parliamentary advantage or doing something that will aid the Democratic Caucus.”

But the preservation of parliamentary prerogatives doesn’t explain all of Reid’s votes. For instance, the Senator had nothing to gain in the first vote of 2006, when he voted for cloture on the Supreme Court nomination of Samuel Alito and then switched his vote to “nay.” The motion carried by a final count of 72-25. Manley offered no explanation for this vote.

Reid also reverses himself more regularly than other Senators in leadership. During five Congressional sessions studied by Roll Call, Reid served as Minority Whip, Minority Leader and Majority Leader. The other Senators who have served in those positions during the same time period — Bill Frist (R-Tenn.), Tom Daschle (D-S.D.), Mitch McConnell (R-Ky.), Dick Durbin (D-Ill.) and Trent Lott (R-Miss.) — reversed themselves a total of 34 times, four fewer than Reid alone.

Senate observers say there are a broad array of reasons for Senators to switch their votes, and a raw tally of reversals doesn’t tell much about the substance of the votes.

Senate Legislative Clerk David Tinsley, who is one of a handful of people who mark the official tally sheets, said very few of the reversals represent an actual change of position by a Senator; the lion’s share are simply mistakes.

“Senators rarely vote from their seats,” Tinsley said. “There is often some chaos when Senators are voting, they are all in the well, they are talking — Senators can easily be distracted” and may make a mistake in their vote. Also, Senate votes frequently are stacked one after the other, and the order of the votes may change at the last minute if an amendment is dropped or added or if a motion to table an amendment is raised. The change in order may create confusion as to what measure is being voted on at the present time, Tinsley said.

Indeed, the vote tallies show the most frequent reversals come on motions to table amendments or bills, where clearly the Senators who are reversing themselves (often en masse) believed they were voting for the underlying bill, not for the motion to set it aside.

Tinsley also said it is possible in very rare cases for the mark on the tally sheet to represent an error by the clerk.

But there also are clearly cases where the mark indicates a vote that switched for a political reason.

In February, 13 Democrats and Sen. Bernie Sanders (I-Vt.) voted against an amendment offered by Sen. Jim DeMint (R-S.C.) to prevent people convicted of terrorism or other felonies from getting access to secure areas of American seaports. But before the vote was over, they all switched to “yea.” DeMint’s office said Sen. Daniel Inouye (D-Hawaii) offered a less restrictive measure that was preferred by Democrats and labor unions, and when leadership made it clear that the Inouye version would replace the DeMint version in conference, Democrats switched to supporting the DeMint version because they knew it was irrelevant. The final vote in favor of DeMint’s amendment was 94-2, but the measure was effectively dead.

Sometimes the motives are hard to determine. On March 22, Arkansas Democratic Sens. Blanche Lincoln and Mark Pryor both initially voted for a proposal by Sen. Mike Enzi (R-Wyo.) to amend the Senate budget resolution to make it harder to pass legislation that would create new burdens for small businesses. Before the voting was done, Pryor and Lincoln had both switched, and the measure failed by a two-vote margin, 47-49.

Enzi spokeswoman Elly Pickett said in an e-mail, “Senator Reid did hold open the vote for an extended period of time for those members to change their votes.” The Senate’s vote database indicates that the next vote began 21 minutes after the Enzi vote began, a bit longer than an average vote in the Senate.

Pryor’s and Lincoln’s offices both insisted the final vote reflected the Senators’ original intentions. “I’m not sure what happened on the floor,” said Lincoln spokeswoman Courtney Rowe. “But we recommended for her to vote ‘no’ and that is how she voted.”

Pryor’s office had a similar recollection. “The Senator’s vote recommendation was always ‘no’; he was always going to vote ‘no,’” said Michael Teague, Pryor’s communications director.

Bill Hoagland, who spent 21 years as staff director on the Senate Budget Committee before becoming senior adviser to then-Majority Leader Frist, said that with moderates such as Baucus and Bayh, it is not uncommon for a Member to differ with the party leadership on a particular vote. In those cases, the Member may switch to support the caucus if their vote is needed, or vote with the caucus originally and switch once the outcome is no longer in doubt. A Senate staffer who asked not to be named pointed out that this kind of tension may be most evident for Senators near the top of the alphabet, whose names are called before the outcome is clear.

Sen. Olympia Snowe (R-Maine) — also in the top 10 with 24 reversals since January 2003 — is a good example on the Republican side, Hoagland said. “She was always down there and Sen. Lott was particularly a friend of hers from their House days. She would cast the vote the way she wanted to vote, but if it made a difference for the outcome to the caucus, she was persuaded by Sen. Lott from his personal relationship — ‘Please take this one for the caucus.’”

Every Senate office mentioned in this article was contacted seeking comment on its boss’s record of reversals, and most were mystified that such a record even existed. None made its Senator available for comment on the matter.

But McCain spokeswoman Melissa Shuffield said the mid-vote reversals are meaningless. “The only vote that counts is the final vote, not the change of a vote within a very short period of time,” Shuffield said, “and the Senator would be glad to defend any vote that he’s taken.”