Mostly the liberals. In 2008 Mitt Romney pushed for ending the favorable tax treatment of employer-based plans (https://www.commonwealthfund.org/publications/fund-reports/2...), which was instantly attacked by the left for "taking away your insurance". Then the ACA set up the marketplace for individual plans which was good, but also inexplicably added employer mandates.

It's the same problem as ending subsidies for higher education and tax deduction of mortgages. In the long run it might be the right thing to do, but in the short term, without offsetting factors, it would cause measurable harm beyond what we should be comfortable tolerating.