Last November, the Clinton campaign issued this following release:

The Clinton Campaign today announced the endorsement of former Democratic National Committee Chair Joe Andrew [...]
Andrew became the youngest DNC chair in party history when he took the reins in 1999, after five years as Indiana Democratic Party Chair. Under Andrew, the DNC rose out of debt, implemented new technologies and grassroots mobilization efforts, and raised more than $225 million.
Andrew is the co-founder and Chairman of the Board of The Blue Fund, a mutual fund which invests in companies meeting standards of social responsibility, environmental sustainability, community participation and respect for human rights.
He is currently a partner with the law firm Sonnenschein Nath & Rosenthal LLP in Washington, DC in the Corporate and Securities, Venture Capital and Public Law & Policy Strategies practice groups.
"Joe was a strong leader who put the Democratic Party on the right path,” Clinton said. "I'm honored to have his support."

Andrew is not a household name, but as a former DNC chair carries great cachet inside the party establishment. As such, he's just become Clinton's biggest nightmare.

A leader of the Democratic Party under Bill Clinton has switched his allegiance to Barack Obama and is encouraging fellow Democrats to "heal the rift in our party" and unite behind the Illinois senator.
Joe Andrew, who was Democratic National Committee chairman from 1999-2001, planned a news conference Thursday in his hometown of Indianapolis to urge other Hoosiers to support Obama in Tuesday's primary, perhaps the most important contest left in the White House race. He also has written a lengthy letter explaining his decision that he plans to send to other superdelegates.
"I am convinced that the primary process has devolved to the point that it's now bad for the Democratic Party," Andrew said in a telephone interview with The Associated Press.
Bill Clinton appointed Andrew chairman of the DNC near the end of his presidency, and Andrew endorsed the former first lady last year on the day she declared her candidacy for the White House.
Andrew said in his letter that he is switching his support because "a vote for Hillary Clinton is a vote to continue this process, and a vote to continue this process is a vote that assists (Republican) John McCain."

His defection to the Obama camp is a disaster for Clinton:

It's a high-profile, high-level signal to other super delegates that it's okay to switch to Obama in order to finally bring about the inevitable conclusion. One got the sense that many Clinton supers were getting antsy at the direction the campaign had taken. The dam was holding, but it has now sprung a leak. The whole thing now threatens to collapse.
It has the potential to give Obama a friendly news cycle for once. He hasn't had many of those lately what with Hillary Clinton, Bill Clinton, Jeremiah Wright, and John McCain all making common cause against Obama.
The math, already near-impossible for Clinton, just got that much harder. It's a net gain of two delegates for Obama -- +1 for Obama, -1 for Clinton.
Attack dog James Carville will likely blow his lid again.

Some people will also stress that Andrew is a Hoosier and can help Obama in his state's primary next Tuesday. You guys know my theory -- that only machine politicians actually deliver any tangible benefits at the ballot box (and that's mostly mayors, though some governors like Ed Rendell also qualify). So I would put this endorsement in the category of Bob Casey or Mike Easley -- they can't hurt, but don't expect many (if any) votes out of it.

But at this point, this is no longer a race about regular votes. Obama will win the pledged delegate, popular vote, and states won counts. The only race left is the one for the supers, and Andrew's defection is probably a fatal blow to Clinton's chances on that front.

Update: And Obama will officially pick up three more supers in Illinois next week.

Update II: And another super, this one in Texas, endorses Obama.