Let's consider what a Republican has to say about it. Edmund G. Ross was a Republican senator from Kansas at the time of the impeachment. He had been a major in the Federal army during the war. Here is what he says:
The ostensible basis of the disagreement which in a few months after the accession of Mr. Johnson to the Presidency began to develop between himself and the Republican leaders in Congress, was the plan of reconstruction put in operation by him during the recess of Congress that year, 1865, and outlined in his North Carolina Proclamation. It availed not, that that plan had been adopted originally by Mr. Lincoln a few days before his death--that it had been concurred in by his entire Cabinet and would undoubtedly have been carried out successfully by him had he lived that plan was made the ground of criticism of Mr. Johnson by the extreme party element in control of Congress...
From: "HISTORY OF THE IMPEACHMENT OF ANDREW JOHNSON PRESIDENT OF THE UNITED STATES BY THE HOUSE OF REPRESENTATIVES AND HIS TRIAL BY THE SENATE FOR HIGH CRIMES AND MISDEMEANORS IN OFFICE, 1868", by Edmund G. Ross (Ross)