It’s rare for a sitting U.S. senator who is not particularly weak to get a serious primary challenger. But that’s exactly what’s happening in Massachusetts, where Sen. Edward J. Markey (D) is now at risk of losing his seat in a primary to Rep. Joe Kennedy (D), who launched his campaign Saturday.

Unlike a lot of Republican primaries that are shaped around ideological purity (or fealty to President Trump), this primary is likely to be defined by one thing: age.

Markey is 73 and has been in Congress longer than Kennedy, 38, has been alive. Their race mirrors the generational debate that is starting to shape the 2020 Democratic primaries. As The Fix’s Eugene Scott notes, a Washington Post-ABC News poll finds that 4 in 10 Democrats say a nominee under 70 has a better chance of beating Trump.

Lesser-polling presidential candidates have tried to score points by attacking former vice president Joe Biden’s 76 years in particular. If they’ve landed, it hasn’t stuck. The three 2020 candidates leading in polling are at least 70. But their broader point, that the Democratic Party needs to nominate someone younger, got a boost when former president Jimmy Carter said this week he didn’t think he could have done the job well at 80.

When there isn’t a presidential race, the U.S. Senate is the place in politics where age is most on display. If Kennedy won the primary and then the Senate race, he would be the youngest currently serving senator; way younger than most. The average age in the Senate is 63, according to the Congressional Research Service. Sen. Josh Hawley (R-Mo.) is the youngest; he was 39 when he was sworn in. Sen. Dianne Feinstein (D-Calif.) is the oldest at 85.

Rep. Joe Kennedy (D) launched his campaign to challenge Sen. Edward Markey (D) for his Senate seat in Massachusetts’s 2020 Democratic primary on Sept. 21. (Reuters)

While many incoming senators are younger — newly elected senators averaged 58 — they aren’t doing much to bring down the average age in the chamber. In fact, the average age has slightly ticked up over the past few years, from 61 six years ago.

More broadly, the diversity of Congress, well, isn’t. This year, a record number of women are members of Congress, and they make up just 24 percent of both chambers. There is a record number of black lawmakers, too, and they make up just 10 percent of Congress, according to the Congressional Research Service. It’s 2019, and Congress is still mostly full of white men over 50.

Lawmakers also tend to be way wealthier than the average American, which is certainly part of Kennedy’s biography as a member of the best-known American political dynasty. By contrast, Markey talks about how he grew up as the son of a milkman.

We’re talking about biography so much in relation to this race because in Massachusetts, this Senate primary won’t be an ideological battle. Unlike Republican primaries, many of which are focused on unwavering allegiance to Trump, Kennedy will have a hard time arguing that Markey isn’t liberal enough on the issues. Markey co-sponsored the Green New Deal with Rep. Alexandria Ocasio-Cortez (D-N.Y.), and she and Sen. Elizabeth Warren (D-Mass.) have endorsed him.

That’s why the Democratic establishment is largely critical of Kennedy’s primary challenge. Why is he doing this now? What’s his campaign pitch if Markey is already sufficiently liberal?

“He’s going to have to spend an awful lot of time justifying why he’s getting in this race, given Markey has effectively served the state for years,” said Democratic Senate strategist Jim Manley.

In his announcement Saturday in Boston, Kennedy framed his Senate candidacy as an answer to President Trump. "“Donald Trump has forced a reckoning in our nation, without question,” Kennedy said. “But to meet this moment, it requires more than just beating him; it requires taking on a broken structure that allowed him to win in the first place.”

Or maybe his name and biography are enough. A Boston Globe-Suffolk University poll shows Kennedy with a lead over Markey.

What he’s attempting to do is extremely rare. Rich Cohen, an author with the Almanac of American Politics, noted to The Fix that since 1990, only eight senators have lost their primaries. The Almanac’s profile of Markey also points out that when Markey had a primary challenger for his Senate seat, he didn’t win overwhelmingly, so his support may be soft. In a state like Massachusetts, where Senate seats don’t come up that often, Kennedy likely figured he had to elbow his way in, or not get in at all.

There are more politically perilous places for Democrats to have a rare competitive Senate primary; whoever wins the primary will be highly likely win the Senate seat.

But Kennedy’s entrance is supremely unhelpful to the national Democratic Party. The Democratic Senatorial Campaign Committee is dedicated to protecting incumbent senators, so if Kennedy does become the front-runner, it may have to spend money to protect Markey. That’s money the committee would much rather be stockpiling to spend in the general election in Colorado and Arizona to unseat Republicans.

Massachusetts doesn’t play into the battle for the Senate. But implicitly, and maybe even explicitly, it will play a big role in Democrats’ broader battle about generational change.