From The Hill :
Paul Rosenzweig, who once served as former President George W. Bush’s deputy assistant secretary for policy at the Department of Homeland Security, and Don Ayer, who served as deputy attorney general in President George H.W. Bush’s administration, were among the signatories.
Rosenzweig told The Washington Post  that the statement was meant to provide “intellectual cover” for other Republicans and help them do the right thing. Most Republicans up to now have defended Trump’s actions.
- “The Hill”: https://thehill.com/homenews/administration/465137-george-conway-conservative-attorneys-call-for-trump-impeachment-vote
- “told The Washington Post”: https://www.washingtonpost.com/politics/george-conway-and-other-prominent-conservatives-call-for-expeditious-impeachment-probe/2019/10/09/4971b404-eade-11e9-9c6d-436a0df4f31d_story.html