By Jason Lange

WASHINGTON (Reuters) – Every two years, a small league of political scientists and economists fire up computer models they’ve developed to predict who will win the U.S. congressional elections in November.

Several who use economic data in their models have concluded that unless the economy gets quite a bit stronger Republicans will not be able to retain their hold on Congress.

The U.S. economy is doing well by many measures. But their analysis, based on data running back to the 1940s or earlier, gives evidence Democrats have a good chance of winning at least 23 seats at November’s midterm polls, enough to take control of the House of Representatives.

One leading model, developed by Michael Lewis-Beck of the University of Iowa and Charles Tien of City University of New York, uses after-tax personal income growth as a key indicator.

That model predicts Republicans will hold on to their majority if incomes grow at least 3.4 percent in the first six months of this year. If it did that, it would be the strongest six-month pace for income growth in two decades.

A burst of income growth is still possible. But data published on Friday suggests Republicans could come up short, with after-tax incomes up a modest 1.5 percent in the six months through May, according to the Commerce Department.

At that pace, their model suggests Republicans will lose 32 seats.

“This just spells trouble for Republicans,” said Lewis-Beck.

Presidential approval ratings are the other critical factor in the model, weighing more heavily than income. With Trump’s rating at 42 percent in Gallup’s early June polls, Lewis-Beck said, “the economy has got to really be rocking for them to overcome that drag.”

In the 2014 midterms, the two researchers predicted Democrats would lose 15 seats. They lost 13. Lewis-Beck and Tien will finalize their forecast in late July.

They are part of a group of top forecasters who will unveil predictions at a political science conference in Boston that begins August…

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