It is very difficult. There are up to 20 ROTC vacancies per year. Each year, WP gives about 45 civil prep offers that will have ROTC nominations that they 99% must get into WP (exceptions to performance). There are also many second time candidates in college with ROTC nominations and high school kids nationwide with JROTC. The competition is very steep.
Service connected nominations like ROTC that don't win those vacancies may come off the NWL, but only for Additional Appointee spots in a ration of 3 congressional:1 service connected. These are the spots between all the mandatory fills and the max class size. That class size is dependent on the upper class retention and the congressional lay mandated 4400 end strength, but don't count on more than about 250. There are approximately 200 that graduate USMAPS every year that often don't win congressional vacancies and class competition goals usually for athletes and minorities that take the remainder of those spots. You would not be eligible for the Qualified Alternate (top 150 that don't win their district) because those are only for congressional nominees by law.
Put it all together, your chances are slim unless you are some truly outstanding candidate that WP can't do without. If that is this case, they'll find you one of those spots.