Michigan is the most competitive open seat on the Senate map this cycle with the retirement of veteran Democratic Sen. Debbie Stabenow. Filing closed last week, and though Republicans have been successful in avoiding messy primaries in many targeted races, the Wolverine State is set to have a crowded GOP contest. Meanwhile, Democrats have largely coalesced behind swing-district Rep. Elissa Slotkin (MI-07).

Former Rep. Mike Rogers, who held that swingy Lansing seat after Stabenow and before Slotkin, is the early frontrunner and has the endorsement of the National Republican Senatorial Committee, much of the GOP establishment and — most crucially — Donald Trump.

But even the former president’s influential backing hasn’t dissuaded other challengers from different factions of the GOP, including former Rep. Justin Amash, venture capitalist Sandy Pensler and physician Sherry O’Donnell. And ultimately, how competitive this contest — which we currently rate as Lean Democrat — becomes may turn on the outcome of the primary and if it devolves into a bruising affair.

The Republican side did get smaller last week when former Rep. Peter Meijer — who

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