Why did Radical Republicans believe that Andrew Johnson would support their agenda?

Radical Republicans thought that Johnson would support their agenda because he had an enduring and passionate hatred for the planter class that had, in his opinion, brought about disunion and civil war. Johnson, a Tennessean, had remained in the Senate when his home state seceded. Since part of the Radical program involved disfranchising the planters that he hated so much, they thought Johnson could be relied upon to support most of their plans for Reconstruction....

Radical Republicans thought that Johnson would support their agenda because he had an enduring and passionate hatred for the planter class that had, in his opinion, brought about disunion and civil war. Johnson, a Tennessean, had remained in the Senate when his home state seceded. Since part of the Radical program involved disfranchising the planters that he hated so much, they thought Johnson could be relied upon to support most of their plans for Reconstruction. However, Johnson had other ideas. He thought that Southerners ought to be in charge of Reconstruction. He thus issued pardons and eventually amnesty to many Confederates, even leaders, and less than a year after the end of the war, many of them returned to positions of leadership.

Johnson also had no sympathy for former slaves. While he of course accepted the Thirteenth Amendment, he was dead set against providing freedmen with voting rights (as Lincoln had openly contemplated for black war veterans near the end of the war). He also held a limited view of the powers of the federal government, and therefore viewed federal action on Reconstruction, like the Freedman's Bureau and the Civil Rights Act of 1866, as unconstitutional overreaches. He vetoed both of these measures. In fact, the Radicals were really born, or at least were mobilized, in the midterm elections of 1866, which saw significant backlash in the North to Johnson's policies.