How did the congressional election of 1866 illustrate the sentiments of the American people regarding Andrew Johnson?

The congressional elections of 1866 were effectively a referendum on Andrew Johnson's presidency. Johnson was deeply unpopular with large sections of the Republican Party over his Reconstruction policies. He was strongly opposed to what he saw as the extremism of the Radical Republican agenda, favoring a more piecemeal, conciliatory approach to incorporating rebel states back into the Union. Unable to close the gaping chasm between himself and Radical Republicans, Johnson used the congressional elections to put his vision of Reconstruction before the American people.
On the campaign trail, Johnson viciously assailed his opponents within the GOP, resorting to the kind of language normally reserved for Southern rebels during the Civil War. In fact, Johnson openly equated the Radical Republicans' alleged extremism with the treachery of Southern secessionists. Johnson's intemperate language, combined with his aggressive demeanor, alienated large swaths of the electorate. The president further damaged both his campaign and his credibility by playing upon racist fears, accusing his Radical Republican opponents of inciting African Americans to murder whites.
When the results came in, they were a disaster for Johnson. Republicans opposed to the president won two-thirds of the seats in both houses of Congress. This meant that not only could anti-Johnson Republicans thwart his legislative program, they could also begin the process that would eventually culminate in his impeachment.