What to watch in tonight's primaries

The September 4 primary is all about Ayanna Pressley who is running in Massachusetts to replace incumbent Democratic Rep. Michael Capuano who has been in Congress since 1999.

Why she matters: She's part of the Alexandria Ocasio-Cortez wing of the Democratic Party; an African-American woman, a political newcomer and a survivor of sexual abuse.

As the New York Times put it: "It’s not a sight you see every day, certainly not around Boston — a black woman mounting a plausible challenge to a 10-term white congressman from her own party, a politician with vast connections who votes the progressive line and opposes everything Trump."

• It's worth noting that Rep. Capuano joined the Congressional Progressive Caucus in his first term in office. This race has really come down to a fight about increasing diversity and electing a new generation of leaders of Congress.

She's raised nearly $900,000, per FEC reports, and in the 36 hours after Ocasio-Cortez's win, her campaign received 205 contributions totaling nearly $18,000 — more than 3x the number of contributions she received in the same period the week before.

P.S. from Washington Post's Dave Weigel: "In four of September's five primary states — Massachusetts, Delaware, Rhode Island and New York — efforts are underway to dismantle the party establishment, starting with long-tenured politicians who first took power when compromises with the right were more routine."