The final U.S. Senate races of the 2020-21 election cycle have continued a pattern that’s emerged over the past decade or so: Senate election results are very much in sync with states’ presidential votes.

Democrats Raphael Warnock and Jon Ossoff won both of this week’s Senate runoffs in Georgia by relatively narrow margins – albeit wider than Joe Biden’s 11,779-vote victory over Donald Trump in the state’s presidential contest two months ago.

With Warnock’s victory over appointed Sen. Kelly Loeffler and Ossoff’s defeat of Sen. David Perdue, 34 of this cycle’s 35 Senate races were won by candidates of the same party that carried the state in the presidential contest. The lone exception was Republican Sen. Susan Collins of Maine, who handily won a fifth term even as Biden won the statewide vote. (Trump did, however, take Maine’s 2nd Congressional District and its one electoral vote.)

After the 2020-21 election cycle, we wanted to update our June 2018 look at the growing alignment between states’ presidential and senatorial votes. We relied on the Federal Election Commission for past years’ election results and compared the winning party in each Senate election with the outcome of the presidential election in that state. The 2020-21 results were taken from The Washington Post’s election tracker.

For analytical purposes, special Senate elections were grouped with the closest midterm election; the two recent Georgia runoffs, which essentially concluded the 2020 election cycle even though they were held a few days into 2021, are grouped with 2020. Three senators elected as independents who caucus, or caucused, with Senate Democrats (Bernie Sanders of Vermont, Angus King of Maine and Joseph Lieberman of Connecticut) were counted as Democrats.

This close relationship between Senate and presidential outcomes should, perhaps, have been anticipated. In a preelection survey by Pew Research Center, only 4% of registered voters in states with a Senate contest said they planned to vote for Trump or Biden and a Senate candidate from the opposing party.

Most Senate races go the same way as state presidential votes

In fact, the vast majority of the regular and special Senate elections held since 2012 – 158 of 176 – have been won by candidates who belonged to or were aligned with the party that won that state’s most recent presidential race, according to a Pew Research Center analysis of election results going back to 1980. That represents a marked contrast with prior years: As recently as 2006, nearly a third of Senate contests (10 out of 33) were won by candidates of different parties than their state’s most recent presidential pick.

The 2017-18 election cycle represented a bit of a departure from the recent pattern, but even then, there were only eight “mismatches” out of 36 regular and special Senate elections – all Democratic victories in states Trump carried in 2016. (The election data we used came mostly from the Federal Election Commission, supplemented by information from the U.S. House Clerk’s office; the 2020-21 results were taken from The Washington Post’s election tracker.)

The alignment of Senate races with presidential voting patterns is a fairly recent phenomenon. In 1980, for instance, Democrats won Senate seats in 12 of the 31 states that held Senate races and were carried by Republican Ronald Reagan. (Reagan won all but six states that year; two of those states elected Republican senators despite going for Jimmy Carter, the Democratic incumbent, for president.) In the 1982 midterms, Democrats won 17 of the 28 Senate contests held in states Reagan had won two years earlier.

The “mismatch rate” – the percentage of Senate races won by an opposite-party candidate to a state’s most recent presidential vote – peaked at nearly 59% in 1986. That year, Democrats won back control of the Senate two years after Reagan’s 1984 landslide reelection, in which he won every jurisdiction except Minnesota and the District of Columbia. Of the 34 Senate races that year, Democrats won in 20 states Reagan had won two years earlier.

Despite uptick in 2018, ‘mismatches’ between Senate, presidential election winners becoming less common

Since the late 1980s, however, the mismatch rate has generally trended lower. In 2012, the same year Barack Obama won 26 states in his reelection, the mismatch rate was about 18%. Of the 38 regular and special Senate elections held in the 2013-14 cycle, all but three mirrored the 2012 presidential vote, for a mismatch rate of just under 8%. (The three exceptions all were Republicans elected to Senate seats in Obama states.) In 2016, all 34 Senate contests tracked the presidential vote in their respective states.

One consequence of the increasing alignment between states’ presidential and Senate voting patterns is a decrease in split Senate delegations. Currently, six states have senators of different parties, the lowest number of split delegations in at least the past 54 years.

The trend also is similar to the decline of split-ticket voting in House races – that is, voting for a House Democrat and a GOP presidential candidate, or vice versa. That development has contributed to the scarcity of House seats “flipping” from one party to the other.

Both the decline in split Senate delegations and in split-ticket voting in House races have been driven by the deep, and stable, divide between Democrats and Republicans on fundamental political values, as well as the fact that Americans are more consistently liberal or conservative in their views than in the past.

Note: This is an update to a post originally published June 26, 2018.

Drew DeSilver  is a senior writer at Pew Research Center.