The last time a president was denied a second term, the omen came a full year before Election Day.

It was back in November 1991 when the supposedly unbeatable Republican candidate, Richard Thornburgh, was crushed in a special election for the Senate. A few days later, Thornburgh went to George H.W. Bush's White House with a warning: "Boys, I'm your canary and there's trouble in the mine."

Enfeebled politically by a worsening economy, Bush would end up losing both Pennsylvania and the presidency in the 1992 election.

Since then, there have been four presidents, and three of them — Bill Clinton, George W. Bush and Barack Obama — were able to win re-election. President Donald Trump is the fourth, his fate to be determined by the voters a little over a year from now. But, like with Bush, a potential omen looms — this time in a Charlotte-area congressional district at stake Tuesday night.

North Carolina's 9th District went for Trump by 12 points in the 2016 election, 54 to 42 percent. But in last year's midterm elections, the Democratic candidate, Dan McCready, battled Republican Mark Harris (who had ousted incumbent Rep. Robert Pittenger in the GOP primary) to a near-draw. And when subsequent revelations of absentee ballot fraud on Harris' behalf invalidated the election, a new race was scheduled. McCready is once again running as the Democratic nominee, this time against Dan Bishop, a Republican state senator.

The limited polling that's been conducted suggests a close contest, and anxiety is high enough among Republicans that Trump held a rally on Bishop's behalf nearby on the eve of the Tuesday special election.

The White House, obviously, will be satisfied with any kind of victory for Bishop, even if it's a close one. In addition, in the wake of the ballot fraud drama and after years of local Republican infighting, Trump and his allies will have some mitigating factors to point to if their candidate's performance is shaky.

But it's also possible McCready will actually win this race. And if he does, it will invite comparisons to that fateful upset in Pennsylvania 28 years ago.

The circumstances back then were much different than they are now.

Bush had in early 1991 led America to a shockingly efficient victory in the Gulf War. In the aftermath of that triumph, his approval rating climbed past 90 percent and he was pronounced invulnerable in 1992. But the economy was stalling and unemployment was rising. By the fall of 1991, Bush's approval wasn't far above 50 percent and there was talk that, just maybe, he'd be in for a real fight for re-election.

In that context, the Pennsylvania special election was clarifying. Thornburgh was a member of Bush's Cabinet — serving as his attorney general — and he was broadly popular in the state after two terms as governor. His opponent, Democrat Harris Wofford, a JFK-era civil rights veteran and former college president, was well-regarded in elite circles but little-known statewide. It looked like a gigantic mismatch, and initial polls confirmed that Wofford was far behind.

Wofford built his campaign on criticism of Bush's economic record and a populist pitch for universal healthcare. Steadily, he gained ground, ultimately beating Thornburgh by 10 points. To the political world, it was dramatic confirmation of a suspicion that had been building for a while: Bush really was in trouble in 1992. And Bill Clinton proved it.

Trump's political standing, by contrast, has been far steadier than Bush's. His approval rating has hovered somewhere between the high 30s and the mid 40s for his entire presidency. His disapproval number is generally above 50 percent. It's been clear all along that Trump will be vulnerable in any re-election campaign. But just how vulnerable? After all, as a candidate in 2016, Trump's poll numbers also looked atrocious, and yet he still found a narrow path to victory.

In that sense, the special election in North Carolina's 9th District on Tuesday could also be clarifying. To win again next year, Trump can ill-afford any slack in the coalition he assembled in 2016. If Republicans can't hang on in a district he carried by double-digits, it will be an unmistakable sign that there's trouble in the mine.