Ronald Reagan clobbered Jimmy Carter in the 1980 New Hampshire presidential election. Four years later, he did the same to Walter Mondale. So resounding were those thumpings, Carter won just two towns in the state, Mondale five.

Republican supremacy in the state did not start with Reagan, nor did it end with him. But Reagan’s two victories may represent the GOP high-water mark in New Hampshire presidential contests. The question now is: Has Republican support in the state bottomed out, or could it continue to fall in 2016? And what might Donald Trump, this year's unconventional GOP nominee, mean for this trend?

To explore these questions, this analysis uses NHPR’s “ElectionStats” database to look at how all 244 cities and towns in New Hampshire have voted in presidential elections, going back to 1972.

Until recently, it was a nearly perfect record of Republican dominance. Even before Reagan, Republicans had only lost the state once in the post-WWII era. And in 1988, George H.W. Bush again won nearly every city and town. When all the votes were tallied, Bush cleaned up 226 of the 238 towns, from one end of the state to the other (see map below).

Dominating though it was, Bush’s victory was the end of an era for New Hampshire Republicans. Come 1992, he was edged out by Bill Clinton, aided by the presence of independent candidate Ross Perot on the ballot. Clinton won the state with just 39 percent, to Bush’s 38 percent.

Whether Bush’s loss was primarily because of Perot or not, the state’s GOP has never recovered its mojo in presidential elections, losing five of the next six contests. New Hampshire Republicans are still competitive in other contests; just look at their majorities in both legislative chambers and their Senate and congressional seats. But 1992 was the beginning of a new era in presidential contests in the state.

Map: New Hampshire's Leftward Shift

Since 1972, New Hampshire has voted increasingly Democratic in presidential elections. That shift has been felt in every corner of the state, with traditionally Democratic cities and towns now even more Democratic, and GOP strongholds shrinking in number and strength. Click on a year to see the statewide breakdown of the presidential vote, and click on any town for local details. (For a better experience on mobile, turn your device sideways.)

Clinton won another three-way contest in 1996 with 49 percent of the state vote. Republicans were back on top in 2000, but barely.

George W. Bush’s squeaker of a win looked nothing like the whuppings of days gone by. By the end of the century, something had clearly shifted in the state’s electorate. Across the state, almost every town was bluer in 2000 than it had been in 1988, with just four towns voting more Republican than they had pre-Clinton.

And the shift was not over yet. Democratic vote margins went up again from 2000 to 2012 in nearly every city and town across the state. It’s as though someone spilled a bottle of blue dye on the state—formerly red towns are now purple-hued, purple towns are now shaded blue, and blue towns are now ever darker.

Today, there are very few deep red Republican strongholds left in a state that used to be dotted with them. Between 1972 and 1988, an average of 167 towns went strong for Republican presidential candidates, giving 60 percent or more of their votes to the GOP. By 1996, just two tiny towns (Hale’s Location and Dixville) went 60 percent pro-Republican -- maybe not surprising, given that the vote split three ways.

But even after Perot exited the scene, Republican strength has not returned. From 2000 to 2012, there were fewer than 30 “strong” Republican towns in each election, with just three in 2008.

Figure: Count of Republican Strongholds, Towns Voting 60% or More Republican

The 1990s in New Hampshire were not a small movement change: It was a political earthquake that detached the state from its Republican moorings and sent it drifting to the left.

The big question is: Why did this happen? The exact causes of the political shift are debatable. In-migration and young voters have clearly played a major role. But the suddenness of the shift challenges explanations that rely on population movement. Others point to changes in the national Republican Party over the era, and the party’s increased emphasis on social issues that have traditionally not played as big a role in New Hampshire politics. Whatever the cause, the shift is unmistakable.

As a result, New Hampshire is now clearly a swing state, hotly contested in every cycle. Though Democrats have won most of the contests, they have all been decided by single digits. That appears unlikely to change in the coming presidential election, at least at this point. A recent WBUR poll found just 2 points separating Hillary Clinton and Donald Trump in the state (Disclosure: I oversee the WBUR poll).

This cycle may be ripe for another third-party upheaval. Both major party candidates are widely disliked by voters, with nearly a third of voters viewing both candidates unfavorably. And little that involves Trump has been ordinary this cycle. For one, he shifts positions so quickly and thoroughly, he is difficult to consistently place on a partisan spectrum.

But so far, polls show familiar battle lines, with partisans on each side mostly supporting their own candidate. If this continues and we end up with an ordinary two-party race, there is nothing to suggest the dark red tide of past decades will again wash across New Hampshire.