The trope and tactic of good cop, bad cop is well known, and it can be useful as an interrogation technique though there are other ways to use the same underlying methods more effectively. It does make for interesting narrative tension.
But this is about the Senate and how each state has two senators. In general, there is a balance and tension formed by the two seats. They are a kind of contrast, a kind of equilibrium exists between them. One is more liberal, the other more moderate, in liberal states and times. Or one is more out there, the other more reserved, personality-wise. Or the one is big on policy and pork, a workhorse, while the other stands out in rhetoric and publicity.
It is through that lens that one should look at someone like Mo Brooks running to replace someone like Richard Shelby.
The long-standing balance of the Alabama Senate delegation was Shelby who is more of the workhorse, head-down type and Jeff Sessions who was more in the mold of shock-jock big-mouths.
That was actually among the factors that made Doug Jones’ reelection efforts harder: he was very much a workhorse-style moderate Democrat. That’s not to say being more liberal would have saved him, but it is to say that one of the strategies that might have helped would be finding a way to somehow counterbalance Shelby.
The why of the phenomenon involves contrast and dichotomy. The electorate needs to be able to tell its senators apart. If a pair of identical twins with identical politics both sought the twin seats for a state, only one would ever stay, even if their politics perfectly aligned and reflected the state. There wouldn’t be enough contrast.
But differentiation is not the only part of it. It also involves power-sharing. The electorate’s various subgroups accept a candidate they don’t want in one seat if it means the other has someone they do want. One picks the restaurant, the other the entertainment, that kind of thing.
If both senators are too much in the same mold, some factions will feel like their position is under threat, that they aren’t getting enough of a say. It’s hard to endure that kind of humiliation if you’re a group that has actual political power: they don’t get to pick both the meal and the movie.
I’m sure political strategists try to find these balances, but it’s much less about finding them than creating them or selling them. If Mo Brooks wanted that seat, he should have modelled his elections and his congressional record to fit that contrast. He didn’t, so while some may think he’s got the seat, his odds aren’t so great.
Now, something else can happen in these situations. Perhaps the shock-jock voting bloc will prefer Brooks’ schtick to Tuberville’s. If so, they might elect Brooks and see to it that Tuberville gets replaced. Or Tuberville might get remade into something that contrasts with Brooks more.
In other states, in other races, these things do happen. A junior senator can become senior for a variety of reasons, and in doing so, in places that prize the familiar, being senior might be enough to keep getting reelected. The contrast onus is on the junior.
(And for the record, part of the reason Donald John Trump was so popular among Republicans was his apparent contrast, perhaps including his weird facial makeup.)