Why did congressional radicals welcome andrew johnson as president first

The Republicans decided that their National Union Party was a place for all loyal men so in 1864 they nominated Southerner and Democrat Johnson for Vice President. Johnson already had quite a reputation, during the secession crisis he had remained in the Senate even though Tennessee seceded. This act made him a traitor to the south and a hero to the north. President Lincoln appointed him Military Governor in 1862 over the state of Tennessee and Johnson used that as an opportunity to make that state a laboratory for his reconstruction plans.