Beto O'Rourke endorsed his former colleague, Rep. Joe Kennedy III, for US Senate on Sun. (CNN)

🚨September is here, and so is the Massachusetts state primary election. Here's a look at what to expect today.

700K+ voters have already voted via early voting, per the Boston Herald. Polls will be open today until 8 pm.

Several high profile races will come to a head: among them, the top-of-the-ballot Massachusetts US Senate race.

Incumbent junior Sen. Ed Markey is facing a primary challenge from four-term Mass. Rep. Joe Kennedy III.

According to the most recent RealClearPolitics' polling average, Markey has an 11.2 point lead over Kennedy.

If he loses, Kennedy would become the first in his family to lose a state race in Massachusetts. (South Coast Today)

Markey hasn't lost a single election since his first race in 1973. Sen. Elizabeth Warren endorsed Markey in July. (WaPo)

Meanwhile, in Mass.’s 1st District, Holyoke Mayor Alex Morse is taking on 16-term incumbent Rep. Richard Neal.

In 2018, Neal fended off a Democratic primary challenge with 70.7% of the vote. He could pull it off again.

Remember Rep. Stephen Lynch from last week's USPS House hearing? Well, Lynch is also facing a challenge from the left.

Dr. Robbie Goldstein, a 36-year-old infectious disease doctor, is mounting a progressive primary challenge against Lynch.

An Aug. Lincoln Park Strategies poll showed Goldstein trailing Lynch by seven points. (Politico)

Don't expect the results to be drawn out: local election officials have four days to report ballot counts. (Boston Herald)