President Donald Trump answers questions from reporters as he meets with Romania's President Klaus Iohannis in the Oval Office of the White House In Washington. August 20, 2019. Kevin Lamarque | Reuters

Only two presidents since World War II — Democrats Harry Truman and Jimmy Carter — have run for reelection in the same year as a recession. While Truman won and Carter lost, history suggests an economic slump would damage Trump's chances in what will already be a tough 2020 race. Incumbents have a harder time winning amid a down economy because they need to find other issues to draw voters, said Lynn Vavreck, an American politics and public policy professor at UCLA. Trump is "better positioned than most" to withstand a slowdown because he ran on identity-focused issues like immigration in 2016, "but without a good economy it will be harder for him to swing marginal voters," she said. Metrics such as the University of Michigan Consumer Sentiment Index help track consumers views' on and expectations for the economy. The metric typically plummets during recessions. Over the survey's history back to the mid-20th century, it has "largely" found that "if the index is low, the incumbent doesn't get reelected," said Richard Curtin, director of surveys of consumers at the University of Michigan. Recession fears climbed this month for a variety of reasons. Trade tensions with China exploded Friday: Trump said U.S. companies are "hereby ordered" to find an alternative to operating in the country, and the Trump administration hiked tariff rates on Chinese goods. A day earlier, a key indicator found the manufacturing sector contracted for the first time in 10 years, adding to concerns about the economy. Meanwhile, corporate profit growth has proven sluggish — though not as bad as some had feared. Trump may not need to worry just yet. Though some early warning signs of a slowdown have emerged, history suggests it could take until after the election in 2021 for a recession to hit. U.S. gross domestic product rose a solid 2.1% in the second quarter. On Wednesday, the nonpartisan Congressional Budget Office projected long-term growth of 1.8%, even though it expects Trump's trade conflicts to drag on economic growth. While 1.8% growth is lower than the long-term U.S. historical average, it would comfortably keep the U.S. ahead of a recession. Uncertainty created by the Trump administration's trade war with China helped to send consumer sentiment lower in August to a level not seen since the beginning of the year, but it still sits well above levels typically seen during recessions. The current sentiment reading would suggest a good case for reelection for an incumbent president, Curtin said.

He notes that Trump's presidency has thrown out some traditional expectations for how voters will react to a president's first term. For instance, views on the economy have become more partisan: Democrats are less likely to have positive feelings about the economy while Trump is in office than they were when Republican President Ronald Reagan was in the White House. Despite the economy's solid standing now, parts of Wall Street see a rising risk of a recession in the next year. Since the possibility has vexed Trump, CNBC looked back at how other presidents fared while running for reelection under the shadow of a recession.

Truman triumphs, Carter falters

U.S. presidents have rarely run for reelection during a full blown recession since the mid-20th century. Truman was the first president after World War II to run for a second term during a recession. He did not actually win a first election — he started 1945 as President Franklin Roosevelt's vice president before taking over when the president died only three months into his fourth term.

History best remembers Truman's 1948 victory for the photo of him gleefully holding up a Chicago Daily Tribune newspaper with the headline "DEWEY DEFEATS TRUMAN" — a projection based on the conventional wisdom that Republican New York Gov. Thomas Dewey would beat him. The result is also significant because he won easily despite a recession. The downturn's timing may explain Truman's success. It did not hit until November 1948, the month of the election, according to the National Bureau of Economic Research. Not only did Truman comfortably win another term in the White House, but also his Democratic Party flipped control of both chambers of Congress. It gained 75 House seats and nine Senate seats. Carter did not have the same luck. The brief recession during his reelection year of 1980 started in January and ended in July — four months before Americans cast their ballots, according to NBER. It followed the 1979 energy crisis, when oil prices spiked and panic about the availability of gasoline spread in the U.S. Carter had encouraged Americans to live more modestly, a push Reagan targeted during his campaign. Republican Reagan trounced Carter, who won only six states. At the same time, the GOP gained more than 30 House seats and a dozen Senate seats. A few factors could explain why an election-year recession appeared to hurt Carter and not Truman. Due to how the government reports economic data, it can take months after a recession starts to confirm it. Voters also may not feel the effects of job losses or lower wages — or a recovery on the other end — until later. In Truman's case, Americans may not have known the recession started when they cast their ballots. When Carter stood for a second term, they may have had no idea that it ended.

Democrats target Trump's economy