There were no whigs left in Congress by 1865. By then they were all "former" whigs. To say that the South would never have been able to rejoin the the Union under a President Hamlin is pure speculation and not well-founded historically. It would be more accurate to say thatIf it had been left to the Whig`s and Republican`s in Congress along with a "President" Hannibal Hamlin who had succeeded President Lincoln after his assassination, the South would have never been able to rejoin the Union. So at the very least, for that we owe Johnson a lot of Gratitude.
the South likely would not have been able to rejoin the Union on terms that left them in control of their former slaves.