Democrats will have a tough job beating President Trump on the economy in 2020. But maybe they don’t have to.

Since Bill Clinton’s upstart 1992 campaign, political strategists have reminded themselves over and over, “it’s the economy stupid.” But pollsters have detected subtle changes in recent years suggesting voters are less likely to give the president credit for a strong economy, and perhaps less likely to punish him or her for a weak one.

“I wouldn’t say the economy has no effect, it’s probably just smaller than it used to be,” Robert Griffin, research director for the Democracy Fund Voter Study Group, tells Yahoo Finance. “We’ve got an incredibly strong economy right now and look at Trump’s approval rating.”

The unemployment rate, at 3.5%, is at a 50-year low, and the stock market keeps hitting new record highs. Yet Trump’s approval rating has been mired in the low 40s and has never broken 50%, according to the fivethirtyeight composite of polls. A strong economy normally pushes a president’s approval rating into the 60s, at least. Bill Clinton’s approval rating was around 60% in the late 1990s, another prosperous era, and Ronald Reagan’s hit 68% in 1986.

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It’s possible the economy matters less to voters when times are good and there’s less to worry about. But there has also been an increase in partisan voting, which means more voters are likely to vote Democrat or Republican for party-identification reasons than they are to vote for whoever they think will be best for the economy.

This helps explain a puzzling diversion in polls on the direction of the economy, as the data below suggests. During the Trump presidency, Democrats have said they think the economy is getting worse, not better, even though data on jobs, wages, GDP and other economic measures show they’re wrong. The economy has, in reality, been getting better. And the biggest improvements have been in coastal areas and big cities where Democrats roost. Dems are either blind to the improvement, or don’t want to acknowledge it.

“Democrats absolutely feel worse,” says Griffin. “They’re most likely to say their personal finances are not doing well, even though they’ve probably improved. They’re less optimistic about the future.”

The outlook on the economy among Republicans, by contrast, has surged during the Trump presidency—which may overstate the health of the economy. Most economic trends under Trump are consistent with the trends during President Obama’s second term, and job growth has actually been slower since Trump took office. GDP growth of around 2% is decent, but when Obama presided over similar growth, Trump and other Republicans called it terrible. The same performance apparently seems much better now that their man is in the White House.