May 2 (Bloomberg) -- Former Democratic National Committee Chairman Paul Kirk formally pledged his superdelegate vote to Barack Obama today, the second former party leader to back the Illinois senator in two days.

Obama ``has and will continue to expand the electorate beyond the traditional Democratic Party base and bring young and new and independent voters to the Democratic banner in November,'' Kirk, a party superdelegate from Massachusetts, said in a statement released by Obama's campaign.

Kirk previously expressed support for Obama, though he hadn't publicly pledged to cast his vote for him at the party's national convention when the nominee will be chosen.

He and former party leader Joe Andrew, who switched his support from Hillary Clinton to Obama yesterday, are among the 795 superdelegates who will have decisive votes at the nominating convention. The endorsements come just days before the May 6 Democratic primaries in North Carolina and Indiana.

Clinton's campaign released a statement saying Kirk has been an Obama backer since at least February and the announcement was intended ``to divert attention from their recent troubles.''

The campaign also released a letter from seven former Democratic chairmen who are supporting Clinton and are urging other superdelegates to do the same.

`Much at Stake'

``We encourage you to continue to fully consider Hillary Clinton and the fact that she is qualified and accomplished,'' the letter said. ``Too much is at stake for us not to consider deeply the choice we must make for our party and our country.''

While New York Senator Clinton still leads Obama in backing from superdelegates -- the party leaders and officeholders who aren't bound by results of primaries and caucuses -- Obama has been catching up since the March 4 round of primaries. With Kirk's declaration, Clinton has 273 superdelegate endorsements to Obama's 250, based on lists provided by the campaigns and public announcements.

Obama leads among pledged delegates, 1,488 to 1,334, according to an unofficial count by the Associated Press. A candidate needs 2,025 to get the nomination.

To contact the reporter on this story: Jonathan D. Salant in Washington at jsalant@bloomberg.net.

Last Updated: May 2, 2008 14:22 EDT