This chart is VERY instructive. When Democrats have held the majority for most of last 80 years - they have had reasonably large to modest margins of control. When Republicans held the majorities, their margins were very slim. It only takes a few "liberal" Republicans to erase the advantage of a majority. The Senate had a famous "gang of five" liberal Republicans that weakened the value of the majority under Bush. The result was continuing heavy domestic spending and liberal public policy - much of which lead to the massive financial crash (which of course is blamed on Bush - along with the wars that followed the 9/11 terrorist attacks - masterminded by a terrorist who Clinton felt was not important enough to capture when he had the chance). Republicans consistently fail to support their party in off year elections or when candidates fail to meet single issue demands of large voter blocks.