Arnold Kling has bothered me in the past for his wild ideas of what progressivism is, but there is cause for hope as well. For example, in this post he talks about reducing the payroll tax and replacing it with a carbon tax, making Social Security less regressive, and shining a spot light on tax expenditures as having a "whiff of liberaltarianism". These are ideas that I hear all the time in the circles I'm a part of in the Democratic-leaning policy analysis community. This stuff that Kling is apparently positive about is standard fare. It's encouraging to see a "whiff of center-left thinking" on such a prominent libertarian blog!
If a bill was introduced tomorrow to cut the payroll tax and replace it with a carbon tax, it would not be the left that would be standing in its way. If Congress got serious about really accounting for the money that gets funnelled out through tax expenditures, it would not be the left that would throw up roadblocks.