To be seized - well stolen is a more accurate term - by the Federal Government is an unconstitutional act. But, when has that ever bothered a liberal/progressive/socialist/dimocrat acting “in the best interest of the nation”?
We, the People, have stood still for many things including, in all probability, means testing for social security. But out right theft of personal funds without any pretext of criminal activity? That's a line that no politician, no matter how ideologically blind, should ever contemplate. Beyond that line live tigers and dragons.
More likely what they will do is require a "small" percentage of IRA and 401ks be put into government bonds or other "socially responsible" investments. After all, since the government is letting us have so much it wouldn't be that big of deal that they get to control 10% (or 25% or 50%) of our investments in tax deferred accounts. < /major sarcasm> Seriously, I fear that that they will start finding a way to either nibble away at it with bad investment choices or convince people to bail out and pay the taxes on early withdrawals.