Three former chairmen of the Securities and Exchange Commission will publicly endorse Democratic Sen. Barack Obama's bid for the presidency Wednesday, including one who served under President Bush.

William Donaldson, who was SEC chairman for about 2½ years from early 2003, along with Clinton and Reagan appointees Arthur Levitt and David Ruder, will join former Fed Chairman Paul Volcker in endorsing Sen. Obama, his campaign said. Mr. Volcker endorsed Sen. Obama in January.

In...