Radical republicans in congress opposed president lincoln's plan for reconstruction because they thought it was too

They felt he was being too soft on the South. They wanted the South to be punished more. They wanted the Federal government to be more in control.

Too moderate

Radical Republicans wanted stronger punishment to be imposed on the South than Lincoln's plan  called for, and wanted a larger role for the federal government in the overall reconstruction plan.  Lincoln's plan called for a path of forgiveness and rebuilding for the South.  His 10 percent plan said that as soon as 10% of a state's voters would swear an oath of allegiance to the Union, the state could be readmitted to the Union.

The answer is ethnic groups.