On its face, President Trump should have a pretty solid chance of reelection. He is an incumbent, and incumbents have a real advantage in presidential elections. Moreover, many economic indicators are positive. Last month, for example, the University of Michigan’s Index of Consumer Sentiment showed that Americans were more positive about the economy than they have been in six months.

But, betting markets put Trump’s odds at 50-50. He’s not a clear underdog, but he’s not in a strong position either. Part of the reason, as I’ve argued, is that he’s not getting much credit for the economy. His approval rating is much lower than we’d expect given how positively Americans feel about the economy.

And so the crucial question is whether his approval rating will increase. Looking at recent incumbent presidents who were elected and then sought reelection, there are three basic patterns. The graph below shows each president’s approval rating in the 10 months leading up to the fall election.

One pattern is exclusive to President Dwight D. Eisenhower, who was very popular to begin with. His approval ratings in the election year were around 70 percent. No president since then has had that luxury when running for reelection.

A second pattern is the one Trump wants to avoid: declining approval in the election year, leading to a loss. This befell both Jimmy Carter and George H.W. Bush. But both dealt with recessions either in the election year or the year before. If a recession doesn’t come in 2020, Trump’s approval rating may not decline as theirs did.

The third pattern is the one Trump needs: increasing approval numbers throughout the election year. Richard Nixon and Bill Clinton are the clearest examples. Nixon’s average monthly approval increased from 49 percent to 61 percent between January and October 1972. Clinton’s increased from 47 percent to 56 percent over the same period in 1996.

Ronald Reagan, George W. Bush and Barack Obama experienced more modest increases, but increases nonetheless. Reagan’s approval had already increased significantly in 1983 as the country recovered from the punishing 1982 recession. But it increased about three more points in 1984 before he was easily reelected.

George W. Bush’s approval rating dropped a few points in the first part of 2004, but then edged up. It was enough for a narrow victory. Obama’s approval rating, which was also relatively stable despite the economic recovery, increased about five points while he was battling Mitt Romney.

As the graph shows, Trump’s approval rating is lower than any incumbent president’s at this point in their first term. And even though Trump won in 2016 despite being viewed unfavorably, it’s a different proposition when you’re the incumbent president. The political scientist Jonathan Bernstein put it nicely:

It’s one thing to vote for someone you dislike, it’s another to vote for someone you think is a bad president. In other words, asking people whether they approve or disapprove of how the president is handling his job is going to be a better predictor of their vote than asking them whether they have a favorable opinion of a candidate.

Of course, Trump’s reelection can’t be ruled out — hence the even odds he has in the betting markets. Nevertheless, his position is unusually weak for an incumbent presiding over a good economy. The simplest strategy for improving his chances is reaching that thin slice of persuadable voters with a message about the good economy.

The question is whether Trump has fully committed to this message. For the moment, the news cycle is consumed with the increasing tensions with Iran — and it is far from clear that Trump’s decision to kill an Iranian military leader, Maj. Gen. Qasem Soleimani, will give him a boost.

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