Justice Democrats, the grassroots group that recruited AOC to successfully challenge Representative Joe Crowley in New York’s 14th Congressional District, has taken heat for breaking the unwritten rule that Democrats are not supposed to take down their own incumbents. So far, the group has endorsed six new challengers for 2020, generally progressives taking on centrists.

But what happens when a progressive challenger takes on a progressive incumbent, and that challenger’s name is Joe Kennedy, grandson of Bobby? Kennedy is likely to launch a primary challenge against Massachusetts Senator Ed Markey. An exploratory committee has mysteriously materialized, along with a July telephone poll asking voters if they would be inclined to back Kennedy over Markey.

Markey, 73, has been a reliable progressive vote but not one of the Senate’s leaders. Kennedy, at 38, who has represented Barney Frank’s old district since 2013, has also been a strong progressive but less than a superstar.

Markey surprised many observers when he became the lead Senate sponsor of AOC’s version of a Green New Deal. That was taken as an attempt to bulletproof himself against attacks from the left.

But a Kennedy challenge to Markey would be less about left-right than about continuing the Kennedy dynasty. Kennedy needs to move fast, since Markey already has two other challengers. If someone else takes the Markey seat, Kennedy’s next chance to move to the Senate depends on Elizabeth Warren getting elected president. Insiders say that Kennedy’s hope is to bluff Markey into retiring.

I’m no fan of dynasties, but neither am I fan of the sacredness of incumbents. These Kennedys—they are very ambitious, they start out callow, and they tend to grow in office. We will probably find out if Joe follows the pattern.