Upvote to Michael_B. I was going to say some similar things but I freely admit I didn't have all those citations handy. Kudos.
But let me add one thing: Why the principle of not punishing congressmen and women for lying is a good idea:
Suppose that there was some legal penalty for lying during a debate in Congress. Suppose a senator or representative could be fined or jailed or kicked out of office or whatever.
This brings up the question: Who determines what is the truth and what is a lie?
Many political debates involve controversy over the relevant facts. People on each side claim that the science or economics or history or whatever supports their position. Often they present totally conflicting "facts". Sometimes because one or the other is blatantly lying. But also because sometimes the truth is complex and hard to determine.
If someone has the power to say that on this controversial question, side A is right and side B is wrong and therefore anyone in Congress arguing in favor of side B is guilty of a crime and will be sent to prison, clearly the people who have this power are the ones who are really in control of the government.
In the worst case, they could be totally partisan and routinely declare that the party or faction they favor is right and the other party is wrong and have all their political opponents thrown in jail, without even bothering to look at the actual evidence. They could lie all they want because they are the ones with the power to declare what is legally considered a "lie", and of course they are always going to say that they are right. Even in the best case, assuming that they are completely honest people trying to be fair, how can you be sure that they will never, ever make a mistake? Especially given that, once they make a decision, no one is allowed to challenge it. If they hear the evidence on some debated question and decide the right answer is X, then no one is allowed to offer any evidence or any argument that in fact X is wrong, under threat of fines and imprisonment.
You can, of course, write on a piece of paper that this "truth commission" will be fair and non-partisan and always examine the best evidence in a totally impartial manner. Now tell me how you insure that it actually happens that way.
The people on this "truth commission" would be the REAL government. Congress and the legislatures would be irrelevant.
As I write this, there is a lawsuit by 17 states (that can be found here and here) against a group of oil companies to charge them with various crimes because they questioned global warming and funded research that challenges global warming. The premise of the lawsuit is that the oil companies lied about global warming. But of course the oil companies deny that they lied: they said they were pursuing the truth. Even if they are wrong, the idea that a court can declare that someone's public statements about a controversial political issue are lies and punish them with criminal penalties strikes at the very heart of democracy. If you're thinking, "but global warming is a proven fact and if they deny it than they are lying", please consider the precedent. A court will decide what statements about controversial issues are legal to make in public and what statements are crimes that can send you to jail. Are you confidant that the courts will always agree with your side? What if a judge came along who said the opposite, who said that he is convinced that global warming is a hoax and anyone who warns against global warming should go to jail. Even if you are absolutely convinced that such a judge would either have to be an idiot or was being paid off by the oil companies or whomever ... are you certain that there will never be a judge who is an idiot or corrupt? Because that's the point: If someone has the power to send you to jail for "lying" about political issues, how do you make sure that that person will never be corrupt and never make a mistake?
Freedom of speech inherently includes the right to lie. Not because we defend lying, but because if the government has the power to put you in jail for lying, then the government has the power to put you in jail for telling the truth.