KALAMAZOO, Mich. (Reuters) - In an early sign of the potential weakness of U.S. President Donald Trump’s path to reelection, two Democratic presidential primary frontrunners are outshining the incumbent Republican in a poll of Michigan voters released on Wednesday.

Slideshow ( 2 images )

Michigan is set to be a key battleground state in the November 2020 election and will be critical to Trump’s hopes of getting reelected to another four-year term.

The poll of likely voters found both former Vice President Joe Biden and U.S. Senator Bernie Sanders leading Trump by 12 percentage points if the vote were to be held today.

Only 36% of likely voters in Michigan said they would vote for Trump, the poll showed.

Michigan, which had not voted for a Republican as president since 1988, shocked political observers in 2016 by backing Trump and helping propel him to the White House.

Winning Michigan would be critical to Trump’s reelection path, along with the states of Wisconsin and Pennsylvania that have similar voting patterns.

The survey of 600 Michigan voters was conducted by Lansing, Michigan-based Glengariff Group by live phone interviews and has a margin of error of 4 percentage points. It was published by The Detroit News and local news station WDIV-TV.

A Quinnipiac University poll of Pennsylvania conducted in early May found Biden leading Trump by 11 percentage points.

Trump’s campaign has signaled it is aware of the warning signs in those states, which have large manufacturing and agriculture sectors that have been affected by the president’s trade wars.

As Democrats must first sort through a more than 20-candidate primary field before concentrating on the general election, Republicans are optimistic they have time to invest in states like Michigan and Pennsylvania and ultimately to have an advantage.

The Michigan poll also matched Trump against three other Democratic presidential hopefuls - South Bend, Indiana Mayor Pete Buttigieg, and U.S. Senators Elizabeth Warren and Kamala Harris. All three also outdid Trump in the poll, although by much smaller margins that Sanders and Biden.