Thaddeus Stevens was a Radical Republican. What did Radical Republicans stand for?
The Radical Republicans thought Lincoln didn't take a hard enough line against slavery.
Thaddeus Stevens was always on the political fringes. He began his political career as part of the Anti-Masonic party, continued with the Whig (Anti-Jackson) party, the Know-Nothing (Anti-Catholic and Anti-Immigrant) Party and ultimately, the (Anti-Slavery) Republican Party.
In Congress, representing Pennsylvania, he was a gadfly to Lincoln for dragging his feet on banning slavery; after the war and assassination, he opposed Andrew Johnson for wanting the southern states back in the Union as quickly as possible. Stevens lobbied hard for Johnson's impeachment. He died in 1868, just in time to see the Republicans quietly drop civil rights for Blacks from their platform.