What a weird proposal this is. Mandatory 6% savings of workers going into a retirement account. Weird because we already have this thing and it's called Social Security. Up the percentage from 6.2 to 9.2 (and matching employer--also, too, eliminate the cap) and then adjust SS payouts accordingly. That has the potential added benefit of immediately boosting current SS recipients' incomes.
The Harkin proposal, on the other hand, is a pretty crappy not-even-second best.