One reason for the large pendulum swings is because Senators have 6 year terms, so in the next election they will face a turn of fortunes. All those Republican Senators who won in 2014 will face an adverse election in the 2020 Presidential election.
While the Democrats have a small advantage, it won't be enough to get anything done, at least for the next several years. It's Congress who writes and introduces new laws, and even during Presidential elections, the Republicans should hold a House majority or 41 Senate seats to filibuster new laws. Republicans may lose one, but they'll keep at least one obstacle. Then in the next midterm election, the Republicans will regain both obstacles.
What are the issues that is defining the political parties?
This is where the Republicans have their strongest hand. A few years ago, there was a populist movement to eliminate the failed Bush tax cuts for the rich that exploded our deficit and lead us into a recession. Now that most of the tax cuts for the rich were eliminated in 2013, there really is no desire to raise taxes. Republicans will use this as their strong hand for many years to come. If Democrats try to raise taxes, the results will be disastrous for them.
Democrats, avoid tax increases at all costs. If you do increase taxes, this will greatly strengthen the Republicans. The best way to seek tax revenue is to find ways to eliminate tax loopholes and controversial tax deductions employed by the rich, as well as putting the pressure on tax cheats and evaders.
This is a controversial issue and a tossup. Republicans like to bring up this topic to portray them as fighting for American citizens over illegal immigrants, but all too often the GOP comes off as being racist and idiotic, and their real agenda is racism and fear that granting citizenship to illegal immigrants will result in more Democratic voters.
Obama seems to have the right approach to the illegal immigration issue, although the Republicans in Congress won't let any of his ideas pass through. Obama's plan is to allow the illegal immigrants who have worked here for a certain number of years to stay without fear of deportation, and offer a pathway to U.S. citizenship as long as they have no criminal record and pay back taxes. Illegal immigrants who have criminal or terrorist ties are deported. Human traffickers are arrested, sentenced, and deported. Those here by illegal human trafficking tactics, such as the abandoned children from Central America last summer, will be deported.
Democrats have the advantage here. In referendum votes across America in 2014, voters overwhelmingly approved of minimum wage hikes. Polls show Americans strongly approve of minimum wage hikes. However, the Republicans still dominated the 2014 elections, despite their opposition to minimum wage hikes. So what to make of this?
While Americans agree the current $7.25 an hour minimum wage is too low, most Americans believe around $10 an hour is more appropriate. Going as high as $15 an hour is too high that will only pass higher prices to the customers, and a very high increase would shock businesses. Also, taxes were a greater issue to the voters in 2014, because near minimum wage workers are in the minority of the American labor pool.
Womens' Rights and Abortion
This is the Democrats' strong hand, but this hand was a disappointment in the 2014 elections. What happened? First of all, there were no Republicans making ignorant, offensive, or dumbass statements like "legitimate rape". Second, most of the Republican candidates of 2014 didn't take a strong right-wing, anti-woman stance. When Democrats tried to air attack ads against Republicans, the Republicans successfully defended themselves against the misogynist charges. When the misogynist charges became a non-issue, voters focused on taxes, jobs, and the economy, which the Republicans had the lead.
For the Democrats, read the section on taxes. This is what hurt you the most in the 2014 elections, and this is what you need to fix for future elections.