He left office in 2003. If I recall, he was the Republican governor who was a Planned Parenthood flak (but isn’t that what “moderate Republican” ALWAYS means?) He was out of office before John Kerry even won a primary! You may as well ask who Christine Todd-Whitman of Arlen Specter is endorsing.
Arlen Specter is pushing daisys since 2012, so I don’t think he’ll be endorsing anyone.