(CNN) In today's Alabama US Senate election, Democrat Doug Jones isn't running just against Republican Roy Moore. Jones is also running against the most powerful trend shaping modern elections for the US Senate.

That trend is a tightening correlation between the way states vote for President and how they vote for Senate. Compared to earlier generations, it has become extremely rare for either party to win Senate races behind enemy lines -- in states that usually back the other side in presidential elections.

Today, Democrats hold just one of the 26 Senate seats in the 13 states that, like Alabama, have voted Republican in every presidential election since 1992. Republicans in turn hold just one of the 30 Senate seats in the 15 states that voted Democratic in all seven presidential elections since then. That means if Jones wins he would be just the third senator from the opposing party to hold one of the 56 Senate seats in the 28 states that have voted the same way in each of the past seven presidential elections.

"The consistency between presidential and Senate results is much, much greater than it used to be," says Alan Abramowitz, a political scientist at Emory University.

Fueling the shift is a growing tendency among voters to see Senate -- and House -- elections less as a choice between individuals than as a quasi-parliamentary referendum on which party they believe should control the majority in Washington. As I've written before, in modern congressional elections, the color on the front of the jersey increasingly matters more than the name on the back.

"I think we are seeing more straight ticket voting -- that voters are making their decision based on what they think about the parties and which party they want to be in control," says Abramowitz. "It's much less about the individuals. 'Who do I want to see represent my state or my district?' is much less important." That's the argument President Donald Trump has made in his support of Moore, who has been accused of pursuing teenagers when he was in his '30s and of sexual assault.

"We already know Democrat Doug Jones is a puppet of Nancy Pelosi and Chuck Schumer, and he will vote with the Washington liberals every single time," says Trump in a robocall recorded for Moore in the final days of campaigning.

That parliamentary instinct isn't insuperable: Democrats Joe Donnelly and Claire McCaskill, for instance, won Senate seats in Indiana and Missouri in 2012 even as the states broke sharply for Republican presidential nominee Mitt Romney, because the GOP picked deeply flawed Senate nominees. But in 2016, every state picked a senator from the same party that it preferred in the presidential race.

Today's Alabama Senate race might offer the most severe test of how far this parliamentary instinct extends. Alabama is one of the nation's most reliably Republican states. And yet Moore was a deeply polarizing candidate even before he was bombarded with the extensive allegations of pursuing very young, and even underage, girls while in his 30s. To many observers, a Moore victory would mark a new peak in the extent to which Senate races now pivot on the party, not the person.

This hardening alignment between presidential and Senate results is solidifying two of the modern Senate's defining characteristics. One is increased polarization. In earlier decades, the Senate's bridge-builders and deal-makers often represented states that voted the other way in presidential races: Think of coastal Republicans such as Mark Hatfield and Bob Packwood from Oregon, or Southern Democrats such as Ernest Hollings of South Carolina and John Breaux of Louisiana. Given that their states usually voted the other way in presidential elections, such senators had an incentive to find bipartisan deals that tempered party conflict.

"In order to stay in office they had to appeal to voters who normally supported the opposing party in presidential races," notes Abramowitz. "So you saw a lot of cross party coalitions, a lot of cross party voting [in the Senate]."

But now most senators are sent to Washington by voters who back the same party in presidential and Senate elections. That gives senators a strong incentive to reliably stand with -- or against -- the president. That dynamic has contributed to the steady increase in party line voting in the Senate since the 1970s.

The other big change rooted in the presidential/Senate connection is increased volatility. Although it's rarely discussed, since 1980 neither party has controlled the Senate for more than eight consecutive years. That's very different than the pattern for nearly a century after the 1890s: From 1955 through 1980, for instance, Democrats controlled the Senate for 26 consecutive years. Earlier in the 20th century, Republicans and Democrats each ran off 14-year streaks of unbroken control.

The difficulty of winning Senate seats in states that usually vote the other way for president has prevented either side from replicating such an extended advantage. In the seven presidential elections since 1992, 24 states have voted Democratic a majority of the time, and 26 states have mostly voted Republican. The inability of either side to consistently win seats across that divide has produced a Senate where the governing party usually holds only a narrow majority --which leaves it vulnerable to quickly losing control even from relatively small shifts in public opinion.

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The tight correlation now evident between presidential and Senate results represents a kind of back to the future pattern.

Early in the 20th century, during another highly partisan era, it was common for each party to dominate the Senate seats of the states that usually supported its presidential candidates. After the consecutive presidential Republican victories in 1900 and 1904, for instance, Republicans held 96% of the Senate seats in the states that had voted for both William McKinley and Theodore Roosevelt. Likewise, after Franklin Roosevelt's first two victories, in 1932 and 1936, Democrats held 89% of the Senate seats in the states that backed him both times.

But through the middle of the 20th century that relationship loosened, as more voters routinely divided their loyalties between presidential candidates of one party and Senate candidates of the other. The relationship unraveled the most from the late 1960s through the 1980s, as Republicans established an extended advantage in presidential contests but generally could not dislodge the tendency of voters to support Democrats for lower-ballot contests, especially in the South. The result was that Republicans controlled only about half the Senate seats in the states that voted twice for either Richard Nixon (in 1968 and 1972) or Ronald Reagan (1980 and 1984).

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But since the 1990s, as Congress has grown more polarized and the ideological distance between the parties has widened, Senate results are again following the presidential outcomes. After 1996, Democrats controlled two-thirds of the Senate seats in the states that voted both times for Bill Clinton. After 2004, Republicans controlled about three-fourths of the Senate seats in the states that voted both times for George W. Bush. And after 2012, Democrats held over four-fifths of the Senate sets in the states that twice backed Barack Obama.

Moreover, in a twist from earlier generations, since the 1990s the party out of the White House has established a dominant hold on the states that reliably vote against the president. After Clinton's two wins, Republicans held about four-fifths of the Senate seats in the states that voted against him both times -- as did Democrats after 2004 in the states that twice opposed Bush. After 2012, Republicans held over three-fourths of the seats in the states that Obama lost twice.

Pulling back the lens for a longer view produces an even more striking picture of how closely Senate and presidential results are now aligning.

Consider the seven presidential elections since 1992. Over that period, 15 states have voted Democratic in all seven elections. Democrats now hold all 30 of their Senate seats except for Susan Collins in Maine. Democrats hold another nine of the 14 Senate seats in the seven states their presidential candidates have won either five or six times since 1992. Two more states have backed Democratic presidential candidates four times, and their four Senate seats are now split.

In all, then, in the 24 states that have supported Democratic presidential candidates in a majority of the past seven elections, Democrats now hold fully 40 of their 48 Senate seats.

Similarly, 13 states have voted Republican in each presidential election since 1992, and the GOP now holds all of their 26 seats except for Heidi Heitkamp in North Dakota. Republicans hold 18 of the 22 seats in the 11 states that have backed GOP presidential candidates either five or six times since 1992. In Florida and Virginia, the states that have voted Republican four times since 1992, Democrats hold three of the four seats, with the latter clearly trending blue since 2008.

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In all, the GOP holds 44 of the 52 Senate seats in the 26 states it has won in the majority of presidential elections since 1992.

The party could deepen that dominance in 2018: In the states that have voted Republican at least five times over that period, Democrats next year must defend Heitkamp in North Dakota, Donnelly in Indiana, Jon Tester in Montana, McCaskill in Missouri and Joe Manchin in West Virginia (not to mention five other seats in states that backed Trump last year). Meanwhile, Nevada's Dean Heller is the only Republican seeking re-election next year in a state that has voted Democratic in most presidential races since 1992.

As Abramowitz notes, the silver lining for Democrats in this scenario is that during midterm elections, Senate incumbents from the party out of the White House don't lose very often -- especially when the incumbent president is unpopular, as Trump is now. But it now qualifies as an anomaly whenever a party wins a Senate seat in a state that usually votes the other way for president. And no Senate upset in recent times would qualify as a greater incursion behind enemy lines than a Jones victory today.