Trump's re-election 'in trouble' as his base deserts him: Fox News polling analyst
September 15, 2015, Donald Trump, 2016 Republican presidential candidate, speaks during a rally aboard the Battleship USS Iowa in San Pedro, Los Angeles, California (Photo by Joseph Sohm/Shutterstock)

"There’s an old adage in polling: 'The incumbent gets what the incumbent is getting.' It means that when analyzing polls, don’t look at the difference between the two candidates — just look at the incumbent’s number. That’s essentially where voters will land on Election Day," Arnon Mishkin wrote in an opinion piece for Fox News Sunday. "The adage is based on the belief that voters have formed a fairly solid opinion about the incumbent, who is typically the better-known candidate. As for folks who say they’re undecided, they’ve usually decided that they’re not supporting the incumbent. But they haven’t thought enough yet to make a final decision. Once they do, they most likely wind up choosing the challenger."

In the case of incumbent president Donald J. Trump, "this year’s presidential race will certainly be one of the contests when the rule works. It’s basically a referendum on Trump."

Current polling shows that although Trump received 55 percent of the vote in 18 states on Election Day 2016, he is now  pulling in 55 percent in only five of the 18. Add to that his trailing numbers in the top 10 states where he received the most support in 2016 and it's a troubling scenario for the former reality star. The 10 states where Trump should be invincible are West Virginia, Wyoming, Oklahoma, North Dakota, Kentucky, Alabama, South Dakota, Tennessee, Arkansas, and Idaho. But he's not. Except for Idaho where he's up only by 2 percentage points, Trump is trailing his 2016 numbers between 5-10.5 percent.

"To be reelected, the president needs to continue to try to boost the turnout from his base," Mishkin wrote. "That means focusing on his core populist messages of immigration and support for economic growth that made him the surprise winner of the 2016 presidential election."