Markets hate uncertainty. It is something that can quickly prompt investors to dump stocks and head for the exits.
Uncertainty is the reason that some investors are getting nervous about the prospect of Donald Trump becoming President.
It is not that Hillary Clinton is loved on Wall Street. Neither candidate is seen as being particularly supportive of free trade, or smaller deficits or a reduction in the size of the government.
But Hillary Clinton is seen as a known quantity. She represents the staus quo. Donald Trump does not.
This is reflected in the donations the two candidates have received from the business sector.
The Wall Street Journal took a look at donations by the chief executives of America's "Fortune 100" companies.
It reports that not a single one of the chief executives has made a donation to the Trump campaign this year. That contrasts with four years ago when almost a third of the "Fortune 100" chief executives donated to Mitt Romney's campaign.
Nineteen of the CEOs did donate to Donald Trump's former Republican rivals, such as Jeb Bush and Marco Rubio.
Eleven of the chief executives have made donations to Hillary Clinton's election bid.
Last month the Clinton campaign received donations from Apple's Tim Cook, American Airlines Group's Doug Parker and Nike's Mark Parker.
Two of the CEOS, including Tim Cook, have encouraged other people to make donations collectively totalling US$100,000. Tim Cook also held a fund raising event for the Republican Speaker of the House of Representatives Paul Ryan.
Hillary Clinton is also winning more support from billionaires.
Bloomberg reports that 17 people on its Billionaires Index have collectively donated over US$21 million to the Democratic nominee. Donald Trump has received just over US$1 million from 12 billionaires.
US election law limits individual donations to no more than US$2,600 for a candidate and $25,000 for a party but donations can also be made to Political Action Committees (PACs).
Hedge fund manager George Soros has made donations of US$11 million to support Hillary Clinton's campaign. Another big supporter is hedge fund owner James Simons, who has made donations of just over US$7 million.
The Trump campaign's supporters include American Homes 4 Rent co-founder Bradley Wayne Hughes Sr. and his daughter Tamara Hughes Gustavson.
The billionaire Koch brothers would normally make big donations to support the Republican Presidential contender. But this time David and Charles Koch have opted to support various conservative Republican Congressional candidates.
Paypal founder Peter Thiel appeared at the Republican convention and urged voters to support Donald Trump. But so far he has not made any donations to the Trump campaign. Instead he has focused his efforts on three local races.
Not everyone on Wall Street is worried about the outcome of the election.
Some investors believe that the checks and balances in the system will restrict the next President's ability to push through any policies that are too radical. This will be more likely if the Congress (House of Representatives and Senate) is split between the two parties.