Sen. Mary Landrieu poses for a photo. (Photo by Rex C. Curry/For The Washington Post)

This post has been corrected.

Sen. Mary Landrieu (D-La.) replaced her campaign manager on Wednesday, just under four weeks before the election. This is not something that campaigns that expect to win handily do -- but it is also not something that dooms a campaign to a loss.

Landrieu is one of several high-profile candidates to revamp her campaign structure this year. Texas gubernatorial candidate Wendy Davis replaced hers in June; House candidate Sean Eldridge fired his in July. These things happen, usually (but not exclusively) when a candidate isn't happy with how the campaign is going. And while candidates usually feel as though their campaigns aren't going well, one assumes that campaigns that see shake-ups in leadership (particularly four weeks before an election) actually aren't.

Looking at campaign manager departures on federal or statewide campaigns since 2000, though, the record is mixed. In 18 cases where a campaign manager was fired, six of the candidates went on to win.

The 18 races we looked at were ones mentioned in news reports between Oct. 1 and Election Day in every federal election cycle since 2000. There's probably some bias in this; campaigns that lose often have turnover in staff mentioned among the reasons. For campaigns that fired or lost campaign managers in October, the record was surprisingly mixed: three of the campaigns went on to win, while five ended up losing.

But the very good news for Landrieu is that three of the four Senate candidates who replaced campaign managers went on to win -- and two of the three winners were incumbents. Sen. Bob Corker (R-Tenn.) fired his campaign manager about 40 days before the general in 2006 but won anyway. Joe Lieberman fired his campaign manager after he lost Connecticut's Democratic primary that same year, but went on to win as an independent. And the campaign manager for Sen. Tom Harkin (D-Iowa) in 2002 "resigned" after being caught taping a strategy meeting held by Harkin's opponent.

We included resignations along with firings precisely because of that Harkin situation. Professional campaign operations often involve resignations, willful departures from campaigns, despite being what anyone in any other circumstance would recognize as a termination. But this means that we also included some resignations that were also actual resignations, usually thanks to untoward behavior on the part of the campaign manager. DUIs for the campaign managers of Rick Renzi (R-Ariz.) in 2002 and Gov. Butch Otter (R-Idaho) in 2006. (Renzi was the only House candidate we looked at to win his race.) Revelations about past misdeeds, as in the case of Gabby Giffords's Republican opponent in Arizona in 2006. Or Mr. Bob Beckel, now a co-host on Fox's "The Five," who in 2002 was helping Alan Blinken's Senate race in Idaho before stepping aside for reasons that you're going to want to read about.

Even though those departures weren't due to incompetence, they still happened, and the effects of switching out the boss still rippled through the campaigns. It's worth noting that many of the candidates that lost, lost badly -- suggesting that they were probably never going to win anyway. On average, the campaigns that won despite campaign manager changes won by about 6.3 points. The ones that didn't lost by 26.3.

By a month out, most of the big decisions have already been made and the campaign apparatus is already in gear: TV ad slots have been bought; GOTV plans are in-hand; most of the mail is scheduled, if not already printed and at the mail house. Meaning that the effect of swapping out campaign managers now might actually be less significant than, say, three months ago. It's the perception that's bad. Of all of the points that we've made here, that's the one that Landrieu already knew.

This post was corrected to reflect that Landrieu's campaign manager was replaced, but is still with the campaign.