Taxing the Little Man

Some Democrats have proposed taxing stock trading in order to help pay for all of the ridiculous spending plans that they have (health care, the original stimulus, another stimulus) and say that somehow individuals won't notice it because "...it’s very small, likely 0.25 percent of each trade." That warms the cockles of my heart...it's small, so I won't notice it!" While on my individual trades I may not notice, I will start seeing it when it hits the mutual funds in my 401(k) and when it hits the broader market and all my stocks go down as a result. Why is it a good idea for the government to get a piece of every transaction whether it's a gain or a loss on top of the extra 15% they take for every gain you have?