A moderate Democrat is someone who sides with the Democratic Party but whose political views tend to fall more toward the middle of the political spectrum. The American political system measures party belief on a left- and right-wing spectrum. Extreme conservatives are on the extreme right, and extreme liberals are on the extreme left, with moderates of both parties falling somewhere in the middle.
In 2004, many of the Democratic members of Congress were considered moderate Democrats. However, as of 2014, moderate Democrats are an extreme minority. This is because people have been polarized by extreme party clashes and have moved further to the left or the right of the political spectrum. The term "moderate Republican" has become much more common because it indicates Republicans who have moved more toward the middle of the spectrum and are, therefore, closer in mindset to the Democratic Party than their colleagues. These are the types of individuals the Republican Party hopes to use to draw more party support from instead of those who may be more likely to side with someone only moderately conservative. Several moderate Democrats simply decided not to seek re-election when their terms are complete, and those who took over the office are either members of the Republican Party or more liberal Democrats.