It's a grandstanding stunt, unfortunately, in an effort to "punish" Disney for perceived wrongs. And because of that it's not going to be a serious thing. Limiting copyright to a flat 56 years period would violate the Berne Convention, which the US is a signatory to (the Berne Convention has a minimum of life+50 copyright for works with an author, or 50 years after publication/first showing for works like films or where the author is unknown).
If they were serious about fixing copyright they'd be talking about revoking the Sonny Bono Copyright Act of 1998, which extended the terms well beyond the minimums required by the Berne Convention. And IANAL but even then as far as I've been told they'd have to let the current copyright extensions put into place by that stupid act expire because of the Takings Clause of the Constitution, so Steamboat Willie will still be under copyright protection until 2024 regardless of anything anyone passes or doesn't pass before the deadline passes.