Why does the economy do better under Democrats?
Aside from policy, there's another reason. Voting is a lagging indicator. Whoever is voted in for president, it happens at least 4 years and 2.5 months before they actually end their term - actually longer because the primary season is almost a year long now.
So, say the economy is doing poorly before the election. People believe Democrats will provide more money to middle class and the poor than Republicans, who are generally the on-your-own party. So, they vote for a Democratic president. But then there are months before that person takes over, and the economy might already be recovering by then, plus the entire 4-year term.
OK, so now it's 4 years later and the economy is booming under a Democratic president. So now many voters figure: "hey, everything's great, but you what? It would be even better if my taxes were lower." At this point, they aren't thinking of stimulus to the economy, because they have jobs and their stock market or income or home values - which can be mortgaged for extra cash - are higher. So, they vote for the Republican who promises to cut taxes. Then, when that Republican gets in, the economy goes into one of its downturns - predictably, but somehow people always think "it's different this time." Now, they're stuck with the Republican tax-cutter who won't stimulate the economy with deficit-spending, and there's a recession.
Now you know why the economy always does better under Democrats than Republicans.