Trump's approval rating drops below 40% in key states that helped him win the election

President Donald Trump's support has dropped below 40% in several key states that helped him win the 2016 election.

An NBC News/Marist poll released on Sunday showed that 36% of voters in Michigan approve of Trump's job performance, while 55% disapprove.

Trump's support was even lower in Pennsylvania and Wisconsin, at 35% and 34% support respectively. In Pennsylvania, 54% of voters disapproved of Trump's job performance, and in Wisconsin, 56% said they disapproved.

The same poll also showed that Democrats hold an edge over a year before the 2018 midterm elections. In Michigan, 48% of voters said they would prefer a Congress controlled by Democrats, compared to 35% who said they want to see the reins remain in Republican hands.

In Pennsylvania, 47% of voters said they want to see Democrats control congress, compared to 37% who supported Republican control, and in Wisconsin, 46% of people said they'd prefer Democratic control compared to 38% who supported Republican control.

Trump's support also appeared to be slipping among key demographic groups that helped deliver him the three crucial states.

The telephone polls were conducted between August 13 and 17, surveying 795 voters in Michigan, 773 voters in Pennsylvania, and 801 voters in Wisconsin, and had a 3.5% margin of error.

While former President Barack Obama won Michigan, Wisconsin, and Pennsylvania in 2008 and 2012, Trump was able to flip the states during the 2016 election, helping put him over the electoral college threshold needed to win the presidency.

Some polling analysts cautioned that since the poll was conducted directly in the aftermath of Trump's initial failure to condemn a violent white supremacist gathering in Charlottesville, Virginia on August 12, it could have exacerbated his unpopularity.

Trump's low approval in several key states that helped him win the 2016 election mirrored other national polls that show his approval rating slumping just seven months into his presidency.

FiveThirtyEight's tracker of Trump's approval rating calculates recent polls while considering their reliability and methodology, and on Sunday showed the president's approval rating at 37.8%. Over 55% disapprove of his performance as president.