President Donald Trump's tit-for-tat tariffs and efforts to shake up America's trade relationships contributed to the GOP losing a majority in the House of Representatives during the 2018 midterm elections, according to a new study that suggests the president's trade strategy effectively backfired at the polls.

Researchers from Dartmouth College's Tuck School of Business, the Center for Economic and Policy Research and the Peterson Institute for International Economics analyzed voter turnout data from the 2016 and 2018 elections. They found GOP enthusiasm fell more significantly in counties that were particularly exposed to the effects of Trump's ongoing trade disputes – namely, agriculture-heavy counties where retaliatory agriculture tariffs against U.S. products were felt most prominently.

The study suggests fallout from the tariffs contributed to Democrats' success in the 2018 midterms, when 43 House seats flipped from Republican to Democrat.

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"We find that greater local exposure to the economic consequences of the trade war was associated with a decline in support and a loss of seats for Republican candidates in the 2018 House elections," according to the report. "This negative association was driven largely by retaliatory tariffs on agricultural products, particularly in political swing counties where Trump narrowly lost the popular vote in 2016."

During his first few years in the White House, Trump has made it a priority to reshape America's trade relationships with its international partners, at various times raising tariffs on or threatening exporters from Canada, Mexico, the European Union and, most notably, China. The efforts were teased as a key pillar of Trump's 2016 presidential campaign, with Trump suggesting that the U.S. was being taken advantage of by other countries as a result of poorly negotiated trade deals.

And although the promises to take China and other trade partners to task helped secure Trump residency in the White House, international retaliation may have cost Republicans five of the more than 40 House seats that flipped from the GOP to Democrats during the 2018 midterms. The report's authors note that Trump's agricultural subsidies helped offset potential losses, but separate research efforts have suggested the subsidies have disproportionately benefited certain farmers in certain regions – primarily large, especially profitable operations in the South.

"In an ancillary calculation, the agricultural subsidies did not appear to prevent the loss of any House seats, despite the estimated positive average effect on overall Republican support," the report's authors wrote in a summary of their findings. "This difference is consistent with the fact that the farm payouts went to a narrow set of rural counties that already tend to vote Republican."

The authors note that their study is "suggestive, not diagnostic," couching their findings by pointing out that "for any seat with a narrow margin, there are many factors that could have tipped the election over the edge." But the findings are not likely to be encouraging for Republicans heading into 2020, given the lingering trade uncertainty with several international trade partners.

The revised North American Free Trade Agreement has yet to work its way through Congress – a delay for which Trump has blamed House Speaker Nancy Pelosi amid the House's ongoing impeachment inquiry. The U.S. has yet to finalize a long-promised partial trade agreement with China, and uncertainty over Brexit and continued trade friction between the U.S. and the European Union has left plenty up in the air heading into a crucial election year.

"Importantly, the potential political influence of retaliatory tariffs was strongest where it may have mattered most – in 'swing' counties where the election outcome had been closest in the 2016 presidential election," the report's authors wrote in their summary.

The report also attempted to analyze how Republican efforts to repeal and replace the Affordable Care Act impacted GOP voter sentiment in 2018. They found that "Republican candidates fared considerably worse … in counties that had experienced larger expansions in health insurance coverage" following the ACA's enactment in 2010.

"This suggests that voters were concerned about losing recently acquired health insurance, consistent with earlier evidence that health care was a highly salient issue in many House races," the authors wrote.