Foster was indeed a moderate Republican, and he was defeated by the radical Ferry in the Connecticut legislature in 1866. But, the differences between moderates and radicals, in this time frame, were as nothing compared to the chasm that opened up between both factions and President Johnson.
During the Thirty-Ninth (1865-67) Congress, both moderates and radicals agreed on the basic Reconstruction program: passage of the Civil Rights Act of 1866, continuation of the Freedmen’s Bureau, no Southern representation without ratification of the 14th Amendment, and when the Southern states refused to ratify, biracial suffrage under the Reconstruction Acts of 1867. Foster (along with almost all Republicans) supported all of these measures; Johnson vociferously opposed them.
Of course, as Nemo notes, by law there would have been a new presidential election in November 1865. Passions, I dare say, would have been rather inflamed in the aftermath of a triple-assassination. I can’t even speculate as to the results.
One further curiosity: by law whomever was elected in 1865 would have been entitled to a four-year term, and Presidential and Congressional elections would have been permanently out of sync.