Jordan Weissman, in a piece titled, "Service Workers Deserve Higher Pay. They Also Desperately Need Some Vacation," which should be enough to tell you that I am totally with him on this, concludes, "if the U.S. does one day join the rest of the developed world and mandate paid vacation days, we ought to make sure part-timers get it too—otherwise we’ll end up making them look even cheaper and easier to abuse in comparison with full-timers."
So if we made paid leave a right, as we damn well should (and I ask, yet one more time, why is the Democratic Party totally MIA on this issue?!), how could we make it applicable to part-timers?
That's easy, actually: just write the law so that workers get at least one hour of paid leave for every X hours of paid work.
For instance, if you want to give every full-time worker two weeks' paid vacation, that's one hour of leave for every 25 hours of work. Scaling up to the year, that's 2000 hours of work and 80 hours of leave, or 50 forty-hour weeks of work and two weeks' vacation.
But if you're a part-timer, working 12 hours one week, 24 hours the next, 18 the next, and so forth, then you would accrue an hour of paid leave every 25th hour you work for a given employer. In the example above, the worker would have accrued her second hour of paid leave 14 hours into Day 3.
The same thing could (and should) be done with sick leave, if we ever write that into the law as well.