The Fix's Aaron Blake explains why Bernie Sanders needs to make up ground with black voters in the Nevada caucuses and the South Carolina primary. (Peter Stevenson/The Washington Post)

There have been a lot of stories in the last week about the “Clinton firewall,” and its supposed erosion. But what is the firewall, and does it still exist in the months since Bernie Sanders became a real challenger to Clinton’s once-presumed waltz to the nomination?

For the uninitiated, the Clinton firewall is a group of states that was supposedly guaranteed to be in the bag for Hillary, mostly due to her dominance in polls of minority voters (black voters in particular — a group amongst which she leads Sanders by a massive margin). Winning those states would put Clinton well on her way to the nomination, and would signal her broad appeal to voters the Democrats are courting in November.

The earliest primary states, Iowa and New Hampshire, are predominantly white — in fact, they’re two of the whitest states in the nation. So those states weren’t great predictors of how Sanders might finish in more diverse states. But with the Nevada caucuses set for Saturday afternoon, and South Carolina looming next Saturday, we’re about to find out just his well Clinton’s “firewall” holds up.

If Clinton does win Nevada and South Carolina and carries a huge portion of the minority vote, she’ll appear to be in firm control of the race. But if Sanders wins Nevada — and especially if he does better than expected among black and Latino voters — it will be a signal that his appeal isn’t as limited to young, white and liberal voters as previously thought.