Political caucus right after assassination

[Angela]

“In a political caucus , held a few hours after the President’s death, “the feeling was nearly universal,” to quote the language of one of their most prominent representatives, “That the accession of Johnson to the presidency would prove a godsend to the country.””

I have come across this quote quite often now and am curious because it is so vague in terms of names and people involved.

It usually is quoted from David Donald who apparently mentioned that the people involved in this caucus were radical republicans.
I have hunted it down to the short biography of Hay/Nicolay, where there is no mention of any party involvement – they just speak of “extreme radicals in Congress”.

I was wondering if the long version of this biography – or maybe some other source – offers more info on the people involved in this caucus and who exactly is quoted in that “godsend” quote???

Help a newbie…?