Both parties have voters that they regularly court and win. Republicans go after and get the social conservatives and the small government crowd. Democrats go after and get the liberals, several minority groups, women's rights, union, etc. BUT neither party actually represents the real interests of these groups. Each party in actuality represents different factions of corporate culture. The republicans represent the defense industry, rich individuals, and private equity. Democrats represent insurance, law firms, and hollywood/music publishing. Neither party acts in your interests. Neither party acts in my interests. When was the last time Republicans actually did something substantial about abortion? It's a huge issue for them nearly every election, and they win elections because of it, but if they actually banned abortion, they couldn't use it as an issue, so nothing happens. Democrats use the union's fear of republican anti-union policies to get union voters, but then consistently do things that are really damaging to worker's well being.
Listen carefully, boys and girls. The US is not a real democracy anymore. It is a reality-tv democracy. The democratic/representative elements are for show. Our so-called representatives are almost always pre-screened. If they are not acceptable to the corporate elite then they don't get campaign money which means they cannot compete in the media for attention. Once you get your final two candidates, it doesn't really matter who you choose. Of course there are some exceptions, occasionally a candidate actually does gather enough grass-roots support to get elected to congress. These people are usually dangerous demagogues, so even then we are screwed.