What had originally looked like a banner year for Senate Republicans is now a cliffhanger. Photo: Photo: John Lund/Corbis via Getty Images

For Democrats, the 2018 Senate elections have been approaching like an Arctic cold front for years. In part the product of two exceptionally good years for Senate Democrats in 2006 (when they gained five net seats) and 2012 (when they gained two more), the 2018 landscape is one of the best in living memory for Republicans. They are defending only nine seats, as compared to 25 for Democrats (two of which — Bernie Sanders and Angus King — are technically independents, who caucus with Democrats). Only one Republican seat up this year is in a state (Nevada) carried by Hillary Clinton in 2016. Ten Democratic seats up this year are in states (Florida, Indiana, Michigan, Missouri, Montana, North Dakota, Ohio, Pennsylvania, West Virginia, Wisconsin) carried by Donald Trump last year. On paper, this should be a year when the GOP makes sizable Senate gains, and indeed, early in the cycle there were elephant dreams of achieving a filibuster-proof 60 votes in the upper chamber.

But a combination of factors, including some unusually popular Democratic senators in red states, and an unusually unpopular Republican president generating stronger-than-normal anti–White House midterm headwinds, has changed the expected dynamics. Thanks to a potential GOP calamity in a special Senate election in Alabama on December 12, Democrats actually have a decent chance of making their own gains in 2018. And there is even a discernible (albeit tricky) path to a net-three-seat gain that would give Democrats control of the Senate. If it happens, that would be a disaster of the first order for the Trump administration and the conservative ideologues who are counting on a Republican Senate to rubber-stamp the president’s Executive and Judicial branch nominees, including (potentially) a fifth vote on the Supreme Court to reverse Roe v. Wade.

Midterm anti–White House dynamics have probably wiped out any fleeting GOP advantage in a number of states that went narrowly for Trump in 2016. According to September data from Morning Consult, Trump’s approval/disapproval ratings are now significantly underwater in Michigan (40/55), Pennsylvania (45/51), and Wisconsin (41/53), where Democratic senators are running for reelection, and also in the Republican-held states of Arizona (44/51) and Nevada (44/51). Some states where Trump remains very popular also happen to have especially popular Democratic senators as well. In West Virginia, for example, Trump’s approval ratio is an impressive 60/36, but Joe Manchin’s, at 53/36, isn’t bad, either. Similarly, Trump is at 51/44 in North Dakota, but Heidi Heitkamp looks even stronger at 55/32. And in Montana, Trump is at 50/45, but Jon Tester is at 53/33. There is no particular reason to think any of these incumbents is in deep peril at the moment so long as the president’s popularity continues to gradually erode. Joe Donnelly of Indiana (47/26) and Claire McCaskill of Missouri (42/39) are looking significantly more vulnerable; not coincidentally, both won six years ago when Republicans nominated unusually weak candidates who imploded after saying stupid things about rape.

At least two Democratic incumbents — Bill Nelson of Florida and Tammy Baldwin of Wisconsin — will be running in highly polarized states with very competitive gubernatorial elections where the campaign could become a long, painful, expensive slog. While Nelson is reasonably popular with an approval ratio of 50/26, he is likely to face Governor Rick Scott, who can self-finance to an enormous extent in a state with many pricey media markets. Baldwin (in dicier territory with an approval ratio of 41/38) may not face as formidable opponent, but her race could be overshadowed by Scott Walker’s attempt to win a third full term as governor.

Meanwhile, Republican problems in their own territory are by no means limited to Alabama. A potentially toxic primary for the Arizona seat of retiring Senator Jeff Flake makes this a strong pick-up possibility for Democrats, who have been steadily gaining strength in the state recently. And the GOP pain could be doubled if poor health forces John McCain to step down before next November. Nevada senator Dean Heller (whose approval ratio is among the worst in the Senate at 39/39) is vulnerable to both a primary challenge and to likely Democratic opponent Representative Jacky Rosen. And a potentially fractious and complicated GOP primary to choose a successor to retiring Tennessee senator Bob Corker could conceivably open a path for Democrats in that unlikely territory, particular if former governor Phil Bredesen decides to run.

Along the same lines, a real wild card for 2018 involves Steve Bannon’s threats to run right-wing insurgent primary challenges to several Republican incumbents who would otherwise be expected to stroll to reelection. The potential targets include Roger Wicker of Mississippi (already opposed by Chris McDaniel, who very nearly defeated Thad Cochran in 2014), Deb Fischer of Nebraska, John Barrasso of Wyoming, and Orrin Hatch of Utah (who may well retire, leaving the seat in all probability to Mitt Romney). These races are all in very safe Republican territory, but at a minimum could drain resources better deployed by the GOP elsewhere.

Nasty Republicans primaries could also cause trouble for the GOP in otherwise-promising Indiana, where U.S. representatives Luke Messer and Todd Rokita are holding a true grudge match; in Arizona, where Establishment Republican Martha McSally and fiery conservative Kelli Ward are likely to collide; and in West Virginia, where Representative Evan Jenkins and state Attorney General Patrick Morrissey are challenging each other’s pro-Trump credentials.

As always, there is the possibility of competitive races emerging that no one currently expects. One possibility involves New Jersey Democratic senator Robert Menendez, who recently underwent a much-publicized corruption trial that ended in a hung jury and a mistrial. Menendez’s approval/disapproval ratio is a dismal 32/41, according to Morning Consult, and the embattled senator could face new charges from federal prosecutors, or at least an ethics investigation by the GOP-controlled Senate. But leading New Jersey Democrats are without exception sticking with Menendez, and so far New Jersey Republicans have not found a credible challenger. Given the strong Democratic lean of the state, it’s no wonder the incumbent is favored to survive.

The big intangible for all 2018 races at every level is turnout. In recent elections Republicans have gained a distinct advantage in midterms as their voting base became aligned with the demographic categories (older, whiter voters) most likely to participate in non-presidential contests, even as Democrats became more reliant on the younger and minority voters most likely to sit out midterms. But if the special and off-year elections of 2017 are any indication, Democratic — or to be more precise, anti-Trump —enthusiasm could erase or even reverse that GOP advantage, at least for this one midterm.

All in all, it’s a Senate landscape that could produce any number of outcomes. At present the Cook Political Report, which is very cautious about its projections, shows ten highly competitive races (including the 2017 special election in Alabama); six are rated as tossups (Democratic-held seats in Indiana, Missouri, and West Virginia, and Republican-held seats in Alabama, Arizona and Alabama), and four more (all currently Democratic) are rated as leaning Democratic (Florida, Maine, North Dakota, and Ohio).

New York’s analysis suggests that if Republicans lose in every state where Trump’s approval rating is below 50 percent, Democrats could gain three seats and win control of the Senate even without Alabama, though that’s a very long shot. But if every senator with a current approval rating under 50 percent lost, Republicans could gain five net seats (an equally long shot). For the obvious reason that only a third of the states are holding Senate elections next year, there is no national polling lens for judging the Senate landscape like the generic congressional ballot, which is highly correlated with the national House popular vote. But if the generic ballot remains as strongly pro-Democratic as it has been during much of 2017 (the Democratic advantage in the RealClearPolitics polling average is currently at 9.3 percent), the likelihood of Democratic gains or at least minimal losses will continue to grow. As Democrats looked ahead at 2018 a year ago, that scenario would have seemed idyllic. And it’s worth making a mental note that the 2020 Senate landscape is skewed toward Democrats nearly as much as this year’s is skewed toward Republicans. So if happy days are not yet here again for Senate Democrats, they could be arriving soon.