It is a very simple concept, really. When the average person gets money, they will spend a portion of it and save a portion of it. The portion that is spent will contribute to current economic performance, but the portion that is saved will contribute to future economic performance. Growth ALWAYS comes as the result of savings and investment.

The government, on the other hand, spends everything it gets. So when the government takes money from people, the portion of the money that would be saved is spent. This action always has the same effect: it increases current performance and retards growth.

To give a numerical example, if the savings rate is 10% among a demographic that the government taxes, you will see 10% of the amount taken in taxes be converted to current consumption from investment. You could argue that the American economy could use some spending right now. You could also argue that with the DJIA at ~8000, we need investment more. In any case, this is the mechanism by which the economy has a short term boom under Democrats.