"A Trump presidency would be enormously bearish for stocks," Gartman told CNBC's "Fast Money" on Monday. "He's a wild card and the market hates wild cards."
The Republican presidential nominee is closing in on Democratic rival Hillary Clinton after her campaign mishandled the release of information regarding a health scare over the weekend, raising questions about the transparency of her campaign. The latest Washington Post and ABC News poll shows that Clinton leads among likely voters by 46 percent while Trump trails at 41 percent. The poll has a margin of error of plus or minus 3.5 percentage points.
The market has reacted negatively to Trump in the past, as volatility tends to spike every time the likelihood of a victory by the GOP nominee increases — and as Gartman views it, it will only get worse as the race gets tighter.