Complex sentence: Lobbying is sometimes referred to as the "fourth branch of government," as they have great influence in U.S. national politics due to monetary resources and the "revolving-door" policy of hiring former government officials.
Split into simpler sentences: Lobbying is sometimes referred to as the "fourth branch of government". Lobbying firms have great influence in U.S. national politics due to monetary resources and the revolving door policy of hiring former government officials.