We're still a long way from President Donald Trump's 2020 re-election campaign, but with both Democrats and Republicans on edge about whether Trump can win again, it's never too early to start looking at his chances.

While most people focus on his approval rating, the best metric for determining Trump's odds of winning a second term may come from looking at consumer confidence in the economy.

In a presentation given at CNBC this week, political strategists Sara Fagen and Doug Sosnik demonstrated that consumer confidence, as measured by the Conference Board, has given indications, going back to Jimmy Carter's presidency, about whether presidents can win a second term.

As you can see in the charts below, when consumer confidence dipped during the third quarter of the third year of a president's first term, the president lost his re-election bid. If consumer confidence rose during the same period, the president was re-elected.

There's a lot of other issues hanging over Trump, so this metric may not hold up. But, if you're looking for a different metric to gauge Trump's future, this might be the one to watch.

Carter and Bush 41, for example, saw consumer confidence fall ahead of their failed re-election bids: