The health scare for the Senate’s longest serving member, Democrat Patrick Leahy, just before he’s due to preside over next week’s impeachment trial was a reminder of just how fragile his party’s Senate majority is.

Leahy is just one of a half-dozen Democratic senators from states with Republican governors. One interim appointment could shift power in an instant in the 50-50 Senate, a circumstance somewhat akin to the one that Democrats dealt with after Ted Kennedy died in August 2009 and Republican Scott Brown’s surprise win in the special election to fill his seat put an abrupt end to the Democrats’ supermajority and nearly took down Obamacare with it.

Things are a little less precarious in the House, where Democrats are clinging to a 221-to-211 majority and David Wasserman with the nonpartisan Cook Political Report says the GOP could use its advantage in statehouses to retake control in 2022 by flipping six seats through redistricting alone.