Clinton backer's defection tightens superdelegate race

• Story Highlights
• Joe Andrew, DNC chairman under President Clinton, endorses Sen. Obama
  
• Andrew: "Heal the rift in this party and unite behind Barack Obama now"
  
• Clinton's lead over Obama among the party's superdelegates now at 19
  
• Several congressional superdelegates announced support for Obama, Clinton
  

WASHINGTON (CNN) -- A Hillary Clinton backer's defection to the Barack Obama camp tightens the race for superdelegates, who could determine which candidate will become the Democratic presidential nominee.

Former Democratic National Committee Chairman Joe Andrew, who was appointed to the post in 1999 by President Bill Clinton, said Friday that it was time for the party to line up behind the senator from Illinois so he can "get about the business of taking on John McCain and make sure we have a Democrat who wins in the fall."

"The only thing that matters is the delegate count. It's just mathematically impossible for Hillary Clinton to pick up enough delegates to be ahead of Barack Obama at the end of this process as well," Andrews said Friday.

Superdelegates -- made up of governors, senators, House members and various other party officials or members -- are also known as "unpledged" delegates. They are free to choose the candidate they like, while pledged delegates are assigned in primaries and caucuses.

With Andrew's switch, Clinton's lead over Obama among the party's superdelegates stands at 19. According to CNN's latest count, Obama has a total of 1,732 delegates (pledged: 1,491, superdelegates: 241). Clinton has a total of 1,592 delegates (pledged: 1,332, superdelegates: 260).

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