Let's start off with what has become a bit of conservative heresy these days: I don't think it's always a bad idea to raise taxes. In fact, because this country has spent so much more than we have, for so long, it's practically a given that we'll have to raise taxes at some point to help make up the shortfall. Some people reject that idea because they misunderstand how the Laffer Curve works, but unless we can get a handle on our profligate spending, there's no question that large tax increases will be in the offing.
Now, all that being said, it would be a terrible idea to raise taxes on anybody right now, whether it's by letting the Bush tax cuts expire or by putting new tax increases into place. Since the Democratic Party doesn't seem to understand this, here's an explanation of why it would be a tremendous mistake to raise taxes right now.
1) Raising taxes when the economy is doing poorly is a terrible idea: There are few things both the Left and the Right seem to agree on these days, but both sides do acknowledge that the economy is doing poorly. In recent months, Democrats have discussed a second stimulus and the Federal Reserve has talked about printing more money because it believes we need to spur economic growth.
Well, the most reliable way to goose the economy is by cutting taxes and the most surefire way to slow economic growth is to raise taxes. Since the economy is already growing so slowly, why in the world would we want to implement a 3.8 trillion dollar tax hike by letting the Bush tax cuts expire? That would be like planting a steel-toe boot to the skull of the economy when it's already on the ground, bleeding.
2) Raising taxes slows long-term economic growth: People respond to economic incentives. So, the larger government gets and the more it raises taxes, the more the economy slows down. The slower the economy moves, the higher the unemployment rate, the less competitive the business environment becomes, and the more people suffer across the board. This factor largely explains why Western European nations are so economically stagnant compared to the United States.