The other educator's answer is spot-on in naming white Southerners as the major opponents and obstacles to Reconstruction. However, they did not succeed in hampering Reconstruction efforts alone. They likely would not have succeeded without the efforts of Northern Democrats as well.
Despite its near demise during the Civil War, the Democratic party made a huge comeback starting in 1874 when they won control of the House of Representatives. This power grab was cemented four years later when they also won control of the Senate. The Republicans had lost a lot of credit as a result of the Panic of 1873, and their cause of Reconstruction began slipping away from them. By this time, enthusiasm for Reconstruction in Washington and throughout the North was severely low. It seemed to most Republicans that the only way to successfully continue it would be a massive military occupation. As it stood, the U.S. Army was far too small and spread too thin to accomplish such an endeavor.
(The entire section contains 2 answers and 544 words.)