The results of U.S. Senate elections increasingly are aligned with states’ party preferences in presidential elections – a trend that could have major implications in this year’s battle for control of the Senate.

Most Senate races go the same way as state presidential votes

The vast majority of the regular and special Senate elections held since 2012 – 122 of 139 – 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.

Even in the 2017-18 election cycle, which represented a bit of a departure from this recent pattern, there were only eight such “mismatches” out of 34 regular and special Senate elections – all Democratic victories in states Donald Trump had 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 presidential faceoff between Donald Trump and Joe Biden may be the marquee event on this fall’s political calendar, but it’s hardly the only one. In particular, the Senate appears very much up for grabs, with Democrats needing a net gain of four seats (three if Biden wins, Sen. Kamala Harris becomes vice president and a Democrat is appointed to succeed her) to wrest control from the Republicans.

Given the stakes, 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 election results and compared the winning party in each Senate election with the outcome of the  presidential election in that state.

For analytical purposes, special Senate elections were grouped with the closest midterm election. 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 year, 35 Senate seats are at stake – 33 regularly scheduled elections, plus special elections in Arizona and Georgia. Republicans hold 23 of those seats, while Democrats, including two independents who caucus with them, hold 12. Given the current 53-47 split in the Senate, Democrats would need a net gain of four seats – or three if the Biden-Harris ticket wins the White House – to take control of the chamber.

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

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 both 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.

Since the late 1980s, however, the mismatch rate generally trended lower, at least until 2018. In 2012, the same year Barack Obama won 26 states in his reelection, the mismatch rate was about 18%. But 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, nine 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.