(CNN) President Donald Trump's main hope for reelection has been the economy. As I noted earlier this week , Trump's approval on the economy has averaged 53% over CNN's last three polls. His overall job approval rating in those same polls among voters has been just 44%.

But what happens if the economy loses some steam? Some indicators, such as the yield curve , suggest this could occur.

Trump's already shaky reelection chances would likely drop significantly.

Right now, the main reason voters approve of Trump's job performance is the economy. A CNN poll from late May found that 26% of those who approve of Trump's job performance said it was mainly because of the economy. That was more than double the next most commonly given answer. Additionally, 8% said jobs/unemployment was the main reason for why they approved of Trump. Among those who disapproved, few said anything related to the economy was the main reason why they disapproved of Trump. For example, only 1% said the Trump tax cuts.

Now, many of those who approve of Trump currently would approve of him regardless of what he did. Still, he'd probably lose some of those who approve if the economy went south. He cannot afford for that to happen. At just 44% approval overall, Trump needs a few lucky breaks even if the economy stays steady. Further, a dip in the economy would probably lose Trump any shot he had at winning over that 9% of voters who approve of him on the economy but not overall.

Remember that when there were signs of an economic slowdown during the government shutdown earlier this year, Trump's approval rating dropped to about 40% with all voters.

Trump's reelection chances could hinge on the state of the economy. Take, for example, job growth.

Job growth is a simple measure and fairly predictive of an incumbent's reelection hopes. I took a weighted yearly average (i.e. from October to October) for each of the final two years before the election and compared this measure to the president's reelection margin in presidential elections dating back to the end of World War II. I say weighted because the growth the year before the election is weighted more heavily.

Applying this to Trump, we find that the average weighted yearly job growth over the last two years has been about 1.5%. This is the type of growth that is consistent with a close reelection margin . Two incumbents with similar job growth over their final two years (George W. Bush in 2004 at 1.1% and Barack Obama in 2012 at 1.6%) won by less than 4 points.

Of course, Trump's current level of job growth is far from a guarantee of winning reelection. Gerald Ford had a nearby 1.9% yearly weighted average growth over his final two years, and he lost by 2 points. It makes sense that job growth isn't a perfect predictor of reelection fortune. Beyond the basic fact that there are plenty of ways to measure the economy, there are issues voters consider besides the economy. For Ford, it was Watergate. For Trump, our May poll suggests it's likely the issues surrounding his character. The state of the economy suggests a range of possible outcomes from moderate sized loss to moderate sized win.

That range of possibilities for Trump becomes a lot more dire if the economy takes a turn for the south. Let's say yearly job growth gets cut in half. That would mean a weighted yearly job growth of less than 1.0%. The only two incumbents running for reelection to face that type of economy since the end of World War II were Jimmy Carter in 1980 and George H.W. Bush in 1992. Both lost by more than 5 points.

In other words, even if voters dismissed concerns about his character, Trump would be an underdog if the economy slides.

For character concerns not to cause Trump trouble, he would probably need yearly job growth to double. That would lift Trump's economy to fit the average of presidents who won reelection by 15 points or more (Dwight Eisenhower in 1956, Lyndon Johnson in 1964, Richard Nixon in 1972 and Ronald Reagan in 1984).

Right now, however, Trump would probably be happy if the economy just stayed the course.