This is becoming a bit ridiculous for Democrats. The $15 is mostly symbolic, with many large employers already paying that much or close to it, so it's just going to squeeze smaller employers in some low cost-of-living areas and won't do shit for their constituents in large blue cities. OTOH, it should be a no-brainer instead of a fixed amount to make it indexed on inflation like it's done in... you know... sane countries.
The bill is indexed to inflation, but due to inflation going up without a min wage increase in the last decade, that's why it starts at $15.
After it hits $15 (over a 4 year period), it's tied to inflation/cost of living.