John Fetterman

An evenly divided Senate. A senator disillusioned with the direction of his own party. A sudden party switch that shifts Senate control, sending shock waves through the institution and American politics itself.

That scenario unfolded in 2001, when then-Senator Jim Jeffords of Vermont stunned his fellow Republicans by bolting from the party to become an independent. He then aligned with Democrats, briefly handing them the Senate majority early in the first term of George W. Bush.

Now, with the possibility of a razor-thin Senate majority looming again after November’s midterm elections, the focus is already on the prospect of party-switchers upending the Senate equilibrium, with the most attention focused on Senator John Fetterman, Democrat of Pennsylvania.

Mr. Fetterman, a Senate nonconformist right down to the hoodie-and-shorts uniform he often wears instead of a suit, has taken harsh aim at elements of his own party, broken with his caucus, avoided party gatherings, developed deep friendships across the aisle and even set up a joint fund-raising committee with his Republican Senate colleague from their home state of Pennsylvania.

He recently published an op-ed chastising his fellow Democrats for having tried to gut the filibuster a few years ago. And he has said that if his party officially turns against Israel, as some of its most progressive voices are urging, it would be a “red line” that would prompt him to leave.

Does all this activity signal that he intends to flee the Democratic Party, pulling the rug out from under Democrats if they manage to net the four seats that would give them the majority next January?

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