In 2016, we learned that political battlegrounds turn up in surprising places. The "blue wall" on which Democrats relied to ensure Hillary Clinton's election — Michigan, Wisconsin, and Pennsylvania — collapsed as populism swept President Trump into office. The upcoming midterms might produce yet another surprising battleground — one that improbably could determine control of the U.S. Senate.

For much of this year, Trump's unpopularity and the normal currents of midterm elections seemed to put Senate control into play, even though Democrats are facing an incredibly tough map. As the GOP faded at the end of the summer in generic congressional balloting, people wondered what seat might end up tipping control in one direction or the other. Would it be Nevada, where incumbent Republican Sen. Dean Heller has to win in a state that Clinton carried? Or would it be Missouri, where Democratic Sen. Claire McCaskill is fighting for her political life? Or North Dakota? Or Tennessee? What state will give us the biggest surprise on Election Night?

Would you believe … New Jersey?

Bob Menendez, the New Jersey Democrat who beat the rap in a corruption trial earlier this year, finds himself in a shocking neck-and-neck race with political upstart Bob Hugin in the deep-blue Garden State. A new Stockton University poll of likely voters shows Menendez only leading 45 percent to 43 percent in a state Clinton carried 55 to 41 in 2016. Another likely-voter poll from Fairleigh Dickinson gave Menendez a six-point lead, but only 43 percent of the vote — a very low number for an incumbent of the state's dominant party. No likely-voter poll has given Menendez more than 45 percent support this year.

That's not a very good omen for a man who won his last election by 19 points. So what's changed? For one thing, FiveThirtyEight's Clare Malone points out, the scandal of the corruption trial has corroded Menendez's standing with voters. He claimed the result vindicated him, but both the Stockton and Fairleigh Dickinson polls put his unfavorable ratings above 50 percent. Almost a quarter of Democratic likely voters in one poll still remain undecided between Menendez and Hugin, who's running as a pro-choice, pro-same-sex-marriage Republican. Undecideds of any stripe tend to break away from the incumbent as the election approaches, and with unfavorables this high, Menendez may be in for a long night.

If that happens, the math makes it almost impossible for Democrats to take control of the Senate. But there may be more than math in play.

In November, Democrats will defend far more seats than Republicans, thanks to an unusually good election cycle for their party in 2006 and a successful defense of those seats in 2012. Republicans will defend nine seats, while Democrats have to defend 26. Ten of those seats are in states where Trump won, most of which have been traditionally conservative or center-right. Early on, the GOP hoped to pick up at least five seats and extend their two-seat margin for the rest of Trump's term. That hope appeared long gone, however, as Democratic voter enthusiasm far outstripped that of the GOP, fueled mainly by outrage over Trump and anger over a lack of leverage in Washington. The common pattern of midterm elections, where the party in power usually loses seats, seemed to be emerging from the polling.

But then the Brett Kavanaugh hearings happened. And suddenly the midterm cycle looks a lot different.

New polling has started showing dramatic shifts in enthusiasm, especially the latest NPR/PBS/Marist poll. In July, Democrats had a 10-point edge over Republicans in rating voting in the midterms "very important." The new data shows almost a dead heat in this regard, with Democrats holding only a two-point edge. Furthermore, the Marist generic ballot poll lead for Democrats has been cut in half, from 12 points to six, a number that approaches the traditional point for a competitive chance for the GOP to maintain or increase its standing in Congress.

Furthermore, NPR points out, it's not just a matter of overall enthusiasm but of specific demographics. And the news is especially bad for Democrats in that regard. "While 82 percent of Democrats say the midterms are very important, that's true of just 60 percent of people under 30, 61 percent of Latinos, and 65 percent of independents," NPR's Domenico Montanaro writes. "If those groups stay home in large numbers, it would blunt potential Democratic gains."

If the catalyst for this enthusiasm is the Kavanaugh confirmation process, it's also a question of where it will be felt most. That will likely be in the red states Trump carried, which means that those incumbent Senate Democrats that seemed safe a few weeks ago might be in real trouble. Two likely-voter polls taken in North Dakota during the hearings put incumbent Democrat Heidi Heitkamp down double digits to GOP Rep. Kevin Cramer after having spent much of the cycle within the margin of error. Democrat Joe Donnelly still leads in Indiana, but he's dropped from 51 percent support in an NBC/Marist poll in late August to 43 percent in two successive Fox News likely-voter polls. In Tennessee, where Democrats hoped to swipe a seat from retiring Republican Bob Corker, Republican Rep. Marsha Blackburn has surged into the lead over former Democratic Gov. Phil Bredesen.

This makes holding onto the New Jersey seat even more critical for Democrats. To do so, however, the party may have to shift resources into the Garden State that would normally go to defending incumbents in less friendly states or to play offense on the few vulnerable Republican seats up for grabs. Even that might not be enough to stave off an angered Republican base in their home states. The Kavanaugh spectacle and Menendez could spell an end to Democratic hopes of seizing control of the Senate, and ironically, with it the confirmation process for judges and other appointees of Trump in the second half of his term.

Editor's note: This article originally misstated Marsha Blackburn's party affiliation. It has since been corrected. We regret the error.