Why should tax rates go down?  Because it will let people keep more of their own money and encourage them to spend, bringing more revenue into the Federal Government.

Imagine you own a T-shirt business.  You make T-shirts for $10 and sell them for $20.  You realize that people aren't buying your T-shirts at $20 so you're not making enough money.  Is the answer to raise the price to $25?  Obviously not!  The answer is to put the T-shirt on sale and offer it at a lower price.  That way you attract more buyers and you make more money.  If you drop the price to $15 you will only make $5 per shirt, but if more people buy it you'll come out ahead.

The same rules apply to taxes.  If they drop the rates the government will see an increase of revenue to the government because people will keep more of their money and will spend it. Every time the government has reduced taxes, the revenue to the government went up dramatically.

If the left really wants to solve our fiscal problems they would stop spening, and put income taxes on sale!