The Christian Post has the story and the direct quote here.
“James Dobson, founder of the conservative Christian group Focus on the Family, reviewed the document and was invited to sign it, but did not, said Gary Schneeberger, a spokesman for Dobson. Dobson consulted the group’s board of directors — a common practice — and the board agreed he shouldn’t sign “due to myriad concerns about the effort,” Schneeberger said.”
“One of the things that disappointed Dr. Dobson was that when the manifesto was initially circulated, no African-American pastors or theologians were on the invite list,” Schneeberger said. “His thinking was, ‘How can this purport to represent the voice of evangelicals when people so vital to who we are as a movement are excluded from involvement?’” . . .
Yes, I am sure that is one of the major reasons Dobson didn't sign (and nothing to do with the fact that he is completely and unashamedly partisan).