Nineteen seats in the U.S. Senate could potentially flip parties if there's an unexpected vacancy, according to Axios' analysis of state vacancy rules, which most often allow the governor to appoint a replacement.

Why it matters: Depending on the senator, a single resignation, retirement or death — by accident or old age — could flip control of the 50-50 Senate, or give Democrats a two-vote cushion.

How it works: In 31 states when there's a vacancy, the governor can appoint someone from any party to serve until the next statewide general election, according to the National Conference of State Legislators.

• In six states, the governor must appoint someone from the same political party as the vacating senator.
• The 13 states that remain all require a special election to let voters decide.
• In eight of those 13 states, the governor can make an interim appointment — from either party — to serve until the special election, which can be months later.

By the numbers: More than a quarter of the Senate is 70-plus. The realities of aging can be unseemly to discuss but don't stop political strategists and party leaders from privately worrying about them.

• Six of those senior senators are Democrats in states where a Republican governor is authorized to appoint a replacement — at least in the interim — if a senator abruptly retires or dies in office.
• At 78, Minority Leader Mitch McConnell is the only Republican senator over 70 who comes from a state (Kentucky) where a Democratic governor would make an appointment.

What to watch: Both Vermont Sens. Patrick Leahy, a Democrat, and Bernie Sanders, an independent who caucuses with the Democrats, will be in their 80s this year.

• Vermont has a Republican governor who can appoint an interim senator from either party, although a special election must be scheduled within six months.
• When Sanders ran for president, and later was rumored as a possible Biden Cabinet member, Vermont Gov. Phil Scott said he would likely fill any vacancy with a left-leaning independent.