The markets are not in a bubble right now, but the price of many assets are expensive thanks to the injection of liquidity by the Federal Reserve, noted economist Nouriel Roubini.
"Across the board I don't see a bubble but I see certainly some frothiness that is not justified by the fundamentals because the economy is barely growing 2 percent, earnings growth is slowing down and PE ratios are now above historical average," the chair of Roubini Global Economics told CNBC's "Closing Bell" on Tuesday.
Assets like public and private equities, real estate, credit and government bonds are all expensive, he noted.
Roubini, who is known as Dr. Doom, said he has always been a realist who looks at the upside and tail risks to the market and economy.
And he thinks the biggest tail risk is a Donald Trump presidency because it will lead to significant policy, political and geopolitical uncertainties.
He also doesn't think the Federal Reserve will hike interest rates at its September meeting later this week.
"There's no reason to do so. The economy is still having mixed data," Roubini said. "You don't want also to rattle the markets right before the election."
However, he thinks a December rate hike is "highly likely."
That thinking is in line with the latest CNBC Fed Survey, where 90 percent of the 41 respondents say the central bank won't raise interest rates in September. But 88 percent believe the hike is coming in December.
—CNBC's Steve Liesman and Silvana Henao contributed to this report.