Complex sentence: McLean considered the clause to be enforceable against the states as well as Congress and considered a tax on passengers entering the United States to be an "impost" and a state tax on such passengers to have the effect of making such imposts non-uniform.
Split into simpler sentences: McLean considered the clause to be enforceable against the states as well as Congress. He considered a tax on passengers entering the United States to be an "impost" and a state tax on such passengers to have the effect of making such imposts non-uniform.