A thief stuck a pistol in a man’s ribs
A thief stuck a pistol in a man’s ribs and said, “Give me your money.” The gentleman, shocked by the sudden attack, said “You cannot do this, I’m a United States congressman!” The thief said, “In that case, give me my money!”This is another political joke. The critical point in this joke is when the thief says "give me my money!" The thief (apparently) thinks that the congressman's money belongs to him, and has decided that robbing the congressman is no longer just petty theft, but also is now an act of justice.
If a person thinks that taxes are too high, and that the government is robbing him, then a congressman would be a good target, since they have the "power of the purse", and can determine how the government spends its money.
That being said, the thief's attack is still misguided, since the congressmen don't themselves get the tax money directly, and that makes it funny.