There tends to be a rough parity between parties in the American political system. That's because both parties shift in response to changing conditions. They go after new groups of constituents, positions on the issues change, and the only rule at the end of the day tends to be that he who can deliver the votes, makes the rules. For example, black Republicans used to vote heavily for the Republican Party and it was the Democrats, not the Republicans, who used to be the biggest advocates of tax cuts.
The biggest change in the political landscape over the next few decades is likely to be America's shift to a majority minority population by 2050. This is a change that appears to benefit the Democratic Party since black Americans currently vote for them by a 9-to-1 clip and Hispanic Americans vote for them 2-to-1. The Republican Party will, of course, adjust to the situation and find ways to bring in more minority voters because it will have no other choice. However, there are other coming trends that don't bode as well for the Democratic Party.
1) The fragmentation of the national media: At one point, the Left had an almost total lock on the media. People watched one of the big 3 liberal networks, they read a liberal newspaper, and they had few other options to find out what was going on in the world. That has changed dramatically in the last few decades. The big three networks? They've lost 55.5% of their audience since 1980. Liberal newspapers? They're dinosaurs that are slowly but surely sliding towards extinction in their current form. Staffs are being laid off across the country and The New York Times, which is the most prestigious paper in America, is 1.1 billion dollars in debt and fading. The further the liberal press falls, the more room there is for the web and talk radio to move into the gap and provide Americans with an alternate viewpoint that's friendlier to conservatism, capitalism, and Christianity.