Jeff Greenfield is a five-time Emmy-winning network television analyst and author.

Only one Republican senator ultimately didn’t vote for the tax bill—and it wasn’t because of concerns about the debt, or the tilt of the bill toward the wealthiest Americans. It was because John McCain was back home in Arizona, battling life-threatening brain cancer.

Mississippi’s Thad Cochran did make the vote, after missing votes throughout the fall, due to a persistent urinary tract infection. The health of the 80-year old Cochran has raised questions about whether he will be able to serve out his term, which has three more years to run.


Should the health of these two senators force them to step down, the political consequences could be hugely consequential. Arizona would have two Senate seats in play in 2018. Democrats have already targeted the seat of retiring Republican Jeff Flake, finding encouragement in the narrow results of Arizona’s presidential contest (Donald Trump won with a 3.5 percent plurality, contrasted with Mitt Romney’s 9-point win in 2012). Capturing both seats could be enough to put Democrats in control of the Senate (assuming they hold all of the seats they’re defending next year—10 of them in states Trump won).

While Mississippi is deep red, Cochran barely survived a 2014 primary challenge from state Senator Chris McDaniel. The Tea Party favorite actually ran slightly ahead of Cochran in the first primary, then lost the runoff by only 7,500 votes. An open seat in Mississippi could trigger an intense fight that could wind up with a fringe candidate sufficiently unappealing to put that safe GOP seat in play. Just ask Alabama.

This speculation might seem morbid, but there’s a point that has to be kept in mind as the 2018 midterms loom. Beyond the traditional measurements—generic ballots, the president’s approval rating, the state of the economy—there are matters of fate that can and have played decisive roles in who takes the reins of power. And in a Senate so narrowly divided, those matters loom especially large; everything from a Supreme Court nomination to the future of heath care to the scope of financial and environmental regulation may hang on a single vote.

Back in 2002, Democrats held a one-vote margin in the Senate, thanks to the defection a year earlier by Vermont Republican Jim Jeffords. (His decision to become an independent and caucus with Democrats shifted the balance of power.) But just 11 days before the midterm election, Minnesota Senator Paul Wellstone died in a plane crash. His stand-in, former vice president Walter Mondale, lost to Norm Coleman, and Coleman became the 51st GOP senator. Had Wellstone been reelected, the Senate would have been evenly split.

One can go back further in time. When the 83rd Congress convened in 1953, Republicans held a 48-47 margin (Oregon’s Wayne Morse had left the GOP to become an independent, but chose not to caucus with either party). Over the next two years, nine senators died. In two of those cases, the deceased senator was replaced by a member of the opposing party. When Ohio’s Robert Taft died in July, his Democratic replacement gave that party a one-vote advantage. The following June, when Wyoming Democrat Lester Hunt committed suicide, his Republican replacement restored a Republican one-vote margin.

The most critical lesson from history is what did not happen. As Senate historian Betty Koed notes, “When the second session began in January 1954, the Democrats actually had a one-member advantage in the body, but the Senate did not reorganize under the Democrats. Or, perhaps I should say, the Democrats did not insist on reorganizing.” This led to an unprecedented situation, best summarized in an exchange between the respective party leaders. When Republican William Knowland said, “I have the responsibility of being majority leader in this body without a majority,” Democratic leader Lyndon Johnson replied: “If anybody has more problems than a majority leader with a minority, it is a minority leader with a majority.”) It’s hard to imagine Democrats making a similar decision today. Back then, both parties were highly diverse, with very conservative Democrats and very liberal Republicans. The party designations were far less reliable predictors of how any given senator would vote.

Not anymore. In recent decades, major votes have broken down consistently across party lines. Not one Democrat voted to repeal Obamacare, or for this week’s tax cuts. Not one Republican voted for Obamacare, or for Bill Clinton’s 1993 budget. Given such near-total partisanship, the loss of one or two senators would shift not just the numerical balance of power, but the balance on issues ranging from judicial confirmations to the funding of the government.

At the risk of taking yet another step into the morbid, here’s another thing to consider: If a senator steps down, how is that senator replaced? While some states require special elections—we just saw that in Alabama—36 states give the governor the power of appointment. And in only four states—Hawaii, North Carolina, Utah and Wyoming—is the governor required to choose a replacement from the same party as the departing senator. Right now, there are 23 Democratic senators from states with Republican governors. (I’m excluding New Jersey from this count because there will be a Democratic governor in a couple of weeks.) Only seven Republican senators come from states with Democratic governors and two of them are from North Carolina, where the governor must choose a Republican successor.

No one but the most fanatical partisan looks at the political horizon in hopes that illness, injury or death removes an opponent from the battle. But neither does it serve a clear-eyed view of politics to ignore how the arrival of the unexpected can radically alter expectations. There have been too many times when a “black swan” has appeared to ignore the possibility—and the consequences— of a sudden twist of fate.