More than a year after he announced his primary challenge, Bobby Kennedy’s grandson has yet to come up with a convincing reason.

Markey, the incumbent, is an effective progressive, if anything to Kennedy’s left. The main difference is generational: But at 73, Markey still has all his marbles and has been the more effective leader and legislative strategist.

The short answer is that Joe, currently a member of the House, thinks he can have the Senate seat because he’s a Kennedy. That’s not good enough. But based largely on name recognition, Kennedy is leading Markey in polls.

There is a time and place for primary challenges of incumbents, but this isn’t one of them. The race will eat up millions in campaign contributions, better spent elsewhere in a crucial election year.

More than a year ago, I got a call from a Kennedy staffer who said that Joe would like to come by and talk with me. No agenda except to talk ideas. The tacit agenda, of course, was to cultivate opinion-leader backing for a not-yet-announced Senate challenge.

I had never spoken with him one-on-one before. Joe was nice enough, but far from impressive.

In fairness, there is a Kennedy family tradition of nepotism, and sometimes it has worked out. Kennedys are often late bloomers. Joe’s grandpa Bobby Kennedy began as a Senate investigator for Joe McCarthy. He became one of our greatest leaders on racial healing, getting out of Vietnam, and rebuilding America.

When Ted Kennedy first ran for Jack’s Senate seat, in 1962 at the tender age of 30, his primary rival, state Attorney General Eddie McCormack (himself from a political family), famously declared that if Ted’s name was Edward Moore rather than Edward Moore Kennedy, his candidacy "would be a joke."

True enough. Something similar could be said of Joe, who is 39. Uncle Ted, who had a very bumpy first few decades in the Senate, matured into the Senate’s most effective progressive. Maybe at 60, Joe will be a great progressive, too.

The difference, of course, is that Teddy was not trying to elbow out an effective progressive incumbent.