Bloom said Trump is completely different than other candidates in that he has no political experience or policy team, and he breaks from the standard GOP party line. That makes uncertainty around his presidency
"massive" and he expects the uncertainty index to be much higher than it is for most first time elections.
"He makes statements. He changes his mind. He has no track record. He has no advisers. He's a blank state right now," said Bloom. That means it could take months to understand his positions should Trump win, and uncertainty could be elevated through next summer as he sorts out positions and works out his relationship with Congress and foreign governments, Bloom said.
"It will probably lead to a drop in investment by large firms and the sector that will be most impacted will be manufacturing because the president has the most influence on trade," Bloom said. He said military, health care and construction sectors could also be impacted.
In the second quarter, GDP grew at 1.2 percent, on the back of consumer spending, which grew by 4.2 percent. Business spending was weak, and there was a 1.2 percent drag from inventories.
Deutsche Bank chief economist Joseph LaVorgna agrees part of the weakness in business spending could already be due to apprehension about the election.
"Nobody's going to invest ahead of the election. Who is going to invest? They're going to wait to see what happens. It's not like the candidates are saying the same thing with the same tax policy. Certainly both parties have totally different ideas of how they're going to approach tax reform," said LaVorgna.
LaVorgna said the hit to the economy could be a couple of tenths of a percent of growth, significant when considering how low the growth rate has been. Instead of investing, he said companies are doing other things with their cash.
"Companies are doing what they've been doing much of the cycle — buying back stock and doing M&A [mergers]. They've been doing it in the equity market but it's really unproductive. It creates a more volatile environment because they're replacing equity with debt," he said.
Lee also said Trump's policies are harder to determine and he therefore creates the conditions for an increase in uncertainty. "If it's a Clinton scenario, it's very close to the current baseline. I would say the uncertainty would be much lower," he said.
Bloom said there is much less uncertainty around Clinton because she's already been in office, and her choice of running mate, Sen. Tim Kaine, is also known.
"Trump is a greater wild card," Lee said.
"One caveat is we don't know what Trump is. … He seems to be someone who places policies out as trial balloons and then renegs on them, and you can't get a fix on where he is. The best spin his people put on it is that everything is a negotiating position. The actual outcome would be much more muted than those kind of outrageous comments would imply," Lee said.
Trump's campaign did not immediately respond to a CNBC request for comment.
Trump has proposed a revamping of the tax structure, in a revenue- neutral plan that would cut taxes for corporations to 15 percent. He would declare a one-time tax rate of 10 percent to encourage companies to bring home cash they are currently holding abroad.
He also proposes a more simplified tax plan for individuals that would have a top tax rate to 25 percent.
Trump also blames U.S. trade policies for a wave of globalization that has ruined U.S. manufacturing, and he says he would bring an economic revival.