Andy Kim should tell Phil Murphy he’s not interested in an appointment to the U.S. Senate.

While the CD-3 congressman turned U.S. Senate candidate is not going to ask me for advice, that’s the best play.

It’s worth recalling how Kim, who was then one of 12 House members from New Jersey, arrived on the statewide scene last fall.

He announced his candidacy immediately after Bob Menendez was indicted in September, only to see First Lady Tammy Murphy do the same in mid-November.

All know what happened next.

Murphy’s candidacy never attracted widespread support while Kim’s did. But for our purposes here, it’s important to understand why.

Kim was an outsider. He was not intimidated by the party establishment support for Murphy.

That of course endeared him to those in the party who were philosophically liberal – those more interested in issues and values than the so-called party pros who just wanted to please the governor by supporting his wife.

Those were the folks who organized and who backed Kim at county conventions all over the state. As was reported at the time, Murphy more or less won only the conventions when party leaders had their thumb on the scale. So when Murphy dropped out in late March, it was a win for Kim and liberal activism.

What made Kim’s fight a movement all its own was his parallel and successful challenge against the “county line” and politics as usual in New Jersey. He won that one too thanks to a federal court ruling.

Now Menendez has been convicted and plans – finally – to resign in about a month. Gov. Murphy will name a replacement to serve the remainder of Menendez’ term, which goes to early January.

Kim has reportedly said he would accept an interim appointment and many Democratic leaders in the state support that idea.

Of course they do.

However, it’s important for Kim to remember how he got to the cusp of the Senate in the first place.

Or in other words, you can’t be a liberal idealist only when you want to be one.

By any objective standard, the governor should not give the open Senate seat to a candidate – any candidate. That makes the candidate an incumbent and gives him an unfair electoral advantage.

Kim would come across as quite the political opportunist if he accepted an interim appointment.

That would hurt his image as a reformer and as a man opposed to the status quo.

It’s also unnecessary. Kim should concentrate on winning the seat for the next six years, not the next five months.

One hears how an interim appointment would help Kim’s seniority, but let’s not go overboard with that. He would still be a freshman senator and as such, low on the seniority totem pole.

Republicans, naturally, want Murphy to appoint a “caretaker” – someone who would occupy the seat until early January and then move on.

A few weeks ago, former Governors Christie and Kean Sr. issued a statement that had history in mind.

It mentioned that when Senate vacancies occurred during their years in office, so-called caretakers got the interim appointment. That would have been Nicholas Brady in 1982 and Jeffrey Chiesa in 2013.

Christie and Kean referred to this practice as a “New Jersey tradition.”

There are a lot of New Jersey political traditions, but this one is a good one.

How rare.