Control of the US Senate is never inconsequential. However, control of the Senate at the end of this election cycle seems to be significantly more consequential than previous cycles. If former Vice President Joe Biden is elected president, his ability to deliver on the Democratic agenda will depend on Democrats taking control of the Senate, and the margin by which they do so. If President Trump is reelected, his administration’s ability to function normally in a second term would improve if Republicans maintain control of the Senate and are able to approve his nominees.

HISTORICAL CONTEXT

Every Senate election cycle, and every individual election within an election cycle, is what academics would refer to as an independent event. Every six years for a given Senate seat, each campaign and its outcome is a function of the individual candidates, the actions of their campaigns, and the current circumstances surrounding those candidates and campaigns. History is important to provide context, but the outcomes are independent from the history.

Nonetheless, history tells us something about odds, and historic experience is illuminating as we look to predict potential 2020 Senate outcomes, because presidential victories, driven by state-specific Electoral College results, tend to align with Senate results too. For example, if a presidential candidate carries Nebraska, chances are high that the Senate candidate of the same party also will win in Nebraska. Going back through election history, four specific trends are valuable for considering the Senate outcome in this election cycle. This is not to say these trends predict the outcome in any specific race, but rather each example speaks to how each outcome would fit, or not, within trends over the last two decades or longer.

1. If Biden wins the presidential election, the White House would change party ownership. The last time the White House changed party ownership and the new owners didn’t also control the Senate was the election of Richard Nixon in 1968. If Biden wins and the Senate is not also under Democratic control, it would be the first time in more than 50 years that an election produces a split White House and Senate.

2. A president that wins reelection does not lose the Senate. This is not to say that a president winning reelection must flip the Senate. Richard Nixon and Bill Clinton won reelection while the Senate remained under control of the opposition party. Since the Civil War, how many times has a president running for reelection lost control of the Senate in the same election? Never. Trump winning reelection and Republicans losing control of the Senate would be a first.

3. In 2016, Republican Senate candidates won every state that President Trump won. Where Trump lost, the Republican candidate lost. The last Republican to win a Senate race when the Democratic candidate for president won that same state was Susan Collins in Maine in 2008. The last Republican to defeat a Democratic incumbent in a presidential election year was John Thune over Tom Daschle in 2004. No Republican Senate candidate has defeated a Democratic incumbent while the Democratic presidential candidate won the same state in the last 20 years. If Biden wins a state, history favors the Democrats holding or flipping the Senate seat.

4. In 2012, four Democratic Senate candidates won states that Mitt Romney won, although none of them defeated an incumbent Republican senator. The last time a Democratic defeated an incumbent Republican senator while the Republican presidential candidate won the state was Mark Begich over Ted Stevens in 2008, and prior to that Mel Carnahan over John Ashcroft in 2000. For Democrats to take decisive control of the Senate (a margin of two or more seats), they will need to win seats that run counter to outcomes over the last 20 years.

With this historical context in mind, below we provide additional context for the Senate seats that could decide how the politics in the Senate will function in 2021.

LIKELY REPUBLICAN

Democrat Doug Jones won election to the Senate from Alabama with a surprising special election victory over the highly controversial Republican candidate Roy Moore. In a presidential election year, with Republicans fielding a significantly less controversial candidate, Tommy Tuberville, Alabama is the Senate seat most likely to change parties in the 2020 election. Count this as a likely Republican pick-up.

LEANING DEMOCRATIC

The three most endangered Republicans in the Senate are Susan Collins (Maine), Cory Gardner (Colorado) and Martha McSally (Arizona). Biden is highly likely to win Colorado and Maine. Thus, for Gardner and Collins to survive will require significant ticket splitting. While McSally has the advantage of Trump being the favorite in Arizona, she is running for election as an appointed incumbent who has consistently polled behind throughout the race. If Republicans win any of these three Senate seats and pick up Alabama, Democrats gaining control of the Senate is unlikely.

THE ONES THAT DECIDE THE MARGIN

If Biden wins the White House and Democrats win the three toss-up races while simultaneously losing Jones in Alabama, one of the following seats must go to the Democrats for them to take functional control of the Senate : Iowa, Montana or North Carolina. These three states are all considered highly competitive. Accordingly, for Democrats to take control of the Senate and expand their working margin for 2021, they will have to defeat Republican incumbents in three states that President Trump won in 2016. At the presidential level, Iowa and North Carolina are toss-ups. The better Biden does in those two states, the more plausible it is for the Democrats to upset the Republican incumbent. For the Democrats to defeat Republican incumbent Steve Daines in Montana in a presidential year would be something atypical in the last generation, as Montana has consistently voted Republican in presidential elections.

THE LANDSLIDE

What does the bottom falling out look like for Republicans in the Senate? If Biden clearly wins the White House, taking as many electoral votes as Obama did in 2008 (somewhere around 365), and is competitive in close races in typically red states, other Republican incumbents could be in danger. John Cornyn (Texas), David Perdue (Georgia) and Lindsey Graham (South Carolina) are all in potential jeopardy in this scenario. But it is worth remembering that, as discussed previously, in the last 20 years only two Republican senators have lost reelection in a presidential election year in which the Republican presidential candidate wins their state, and both of those instances occurred under unusual circumstances.

NOT COMPLETELY OFF THE TABLE

There are two races on each side that at least warrant a passing comment because the outcome is not the certainty that we see in other Senate races that are considered safe seats. On the Republican side, the open seat in Kansas and, of course, Mitch McConnell’s seat in Kentucky are seats Democrats dream of carrying on November 3. While not impossible, those Senate seats changing hands seems improbable. Alternatively, in a world where Republicans have a better day than conventional wisdom might expect, Democrats would be concerned about Gary Peters (Michigan) and Tina Smith (Minnesota). Of the four mentioned here, Peters is the most plausible upset because of the potential that Trump could win Michigan.

A RUNOFF TO WATCH

It is highly unlikely that the winner of the Georgia special Senate election will be decided on November 3. Absent the unlikely occurrence that one of the six candidates wins a majority, it will necessitate a runoff election. The outcome of that runoff will determine the final Senate margin and will occur in the aftermath of the general election, which will make the politics of that vote an event unto itself.