We may have already reached a critical juncture in the 2018 Senate midterm elections, so beware of simplistic interpretations. Too often people look at possible electoral outcomes in a binary way; everything is either a zero or a one. In this case, it’s either Democrats cannot win a majority in the Senate, or Republicans will lose their majority. Many people turn a deaf ear toward nuances and “what if’s.”

Until now, it was difficult to see how it would be possible for Democrats to win a majority. It’s still a steep climb, but if former two-term Democratic Gov. Phil Bredesen runs in Tennessee, as he may well do, Republicans could plausibly lose their majority. Bredesen is expected to make a decision in the next few weeks. Should he run, the race for Tennessee’s Senate seat becomes a toss-up. If he doesn’t, it will likely remain Republican.

As Cook Political Report Senior Editor Jennifer Duffy puts it, “The job for Republicans was to keep it from getting mathematically possible that the Senate could go Democratic.” Duffy, who oversees both Senate and gubernatorial coverage for the newsletter, believes that until now the GOP controlled the math.

Republican Sens. Jeff Flake of Arizona and Dean Heller of Nevada are facing more difficult fights than any of the 25 Democratic incumbents up for reelection, even the 10 in states that Donald Trump won last year, and specifically the five that Trump won by 19 points or more. So up to two Republican Senate seats are already in real jeopardy.

With a level political playing field, Democrats could be expected to lose several if not a handful of seats. But with Trump’s approval rating hovering at 37 percent and the GOP Congress in unusually low standing, the playing field is not level and doesn’t figure to be anytime soon. Keep in mind that in four of the last six midterm elections, one party or the other lost their majorities in either the Senate or the House or both. The only exceptions were in 2002, 14 months after the 9/11 attacks, and in 1998, which amounted to a backlash against Republicans over President Clinton’s impeachment.

But with Democrats needing a three-seat net gain to capture a majority, there really didn’t seem to be a third Republican seat in jeopardy—at least until two-term Sen. Bob Corker announced his retirement on Sept. 26 and it became clear that Bredesen was seriously considering a bid. Suddenly, the third Democratic pickup doesn’t seem so far-fetched.

A three-seat net gain for Democrats is still pretty unlikely. They would have to hold onto all 25 of their own seats up for reelection, then win in Arizona, Nevada, and Tennessee. The only alternative paths would have to include winning either the special election in Alabama coming up in December or beating Sen. Ted Cruz in Texas next year. While an Oct. 14-16 Fox News poll showed the Alabama race between former state Supreme Court Chief Justice Roy Moore (R) and former U.S. Attorney Doug Jones (D) tied at 42 percent each, other polls are showing an unimpressive single-digit lead for the controversial Moore.

My feeling is that Moore is an extremely weak candidate, the most beatable person Republicans could have possibly come up with, and that Jones is a better candidate than I would have expected Democrats to field. But Alabama is still Alabama, arguably the toughest state in the country for Democrats in statewide federal elections. It’s easy to see how Democrats can get close in Alabama, but an outright victory looks much tougher. And in Texas, while Cruz certainly has his detractors, he has addressed many of his political problems and is a strong favorite next year. Should Bredesen run, that race seems infinitely more plausible than either Alabama or Texas.

Bredesen made a fortune in business before entering politics, and then served two terms as mayor of Nashville (1991-1999) and two terms as governor (2003-2011). It’s hard to see how any other Democrat could win in Tennessee, even in a good year for the party. The likely Republican nominee is Rep. Marsha Blackburn, who just had a really bad week after it was revealed that she was one of the principal backers of a law that allegedly hampered the Drug Enforcement Administration’s ability to restrict the sale and distribution of opioids. The 60 Minutes/Washington Post exposé has already cost Rep. Tom Marino his appointment as Trump’s drug czar. This story will likely blow over, but in the current environment it makes for an intriguing matchup: a candidate who has served in Congress for the last 14 years for a party now in the doldrums against against a successful mayor and governor untainted by Washington.

Many, though not all, Republican problems in Arizona and Nevada are self-inflicted. Trump has attacked both Flake and Heller, and each is facing Trumpian GOP primary challengers. They could either lose to less electable Republicans, or emerge weakened after bare-knuckled primary fights. Both states are toss-ups. Hard-core conservatives and the alt-right, led by former Trump adviser Steve Bannon, are seeking to purify the party of establishment figures and even conventional conservatives, and this effort is threatening the GOP majority in a big way.

So watch Tennessee very carefully over the next few weeks. Bredesen’s decision, combined with Corker’s earlier decision to abide by his pledge to serve only two terms, make the Volunteer State the pivotal one if Democrats are to have any plausible path to a Senate majority.

This story was originally published on nationaljournal.com on October 24, 2017