Still, it's not impossible. In 1980, John Anderson pulled out of the Republican primary and ran unsuccessfully as an independent, appearing on all 50 states' ballots. In 1992, Ross Perot mounted a third-party bid that had him on all 50 states' ballots. In 2012, Gary Johnson began as a Republican candidate, but ended up running as a Libertarian -- making the ballot in 48 states and the District of Columbia.
Trump will have to keep an eye on the calendar and make some strategic decisions on where he chooses to run if he wants to mount a third-party bid, said minor party activist and ballot access expert Richard Winger.
Winger, a Libertarian, has spent 40 years advocating for minor party candidates and maintains a website
about ballot access.
To appear on ballots in all 50 states and the District of Columbia would require collecting about 570,000 signatures, he said.
According to a chart published in the July issue
of his newsletter, the earliest deadline Trump would have to meet would be May 9 to get on the ballot in Texas. But Winger said a candidate could feasibly mount a legal challenge to that date, because Texas doesn't require independents for other offices besides president to file until June.
Some other states also ostensibly have earlier deadlines, but those are being challenged in court, and previous Supreme Court decisions have supported that independent presidential filing deadlines should be summer at the earliest.
Other than Texas, four states have deadlines in June, and the rest are in July, August and September.
What may be more of an issue, though, are so-called sore loser laws, which essentially prohibit candidates from running in a party primary and then a general election as a candidate not from that party.
According to a 2011 Georgetown Law Review article
by Emory associate law professor Michael Kang, all but three states have such laws on the books -- though Kang's research was focused more on congressional elections, he told CNN in an email.
"The laws for presidential elections are sometimes different and I think always in the less restrictive direction," Kang said. "Even some states with outright prohibitions on sore loser candidates for Congress or other offices have an exemption for presidential candidates. So applied to Trump's case, there's a lot less in the way of an independent run for a president than there would be for Congress or most other offices."
But Winger cautioned that it's not as cut and dry as 47 states disallowing the move, with each state wording the laws differently.
For example, while it has been reported that Michigan would ban a third-party run, Winger pointed out
that the law doesnt prevent a run as an independent, just one under the guise of another party. In Ohio, it's the opposite, added Mark Brown, chair at Capital University Law School and an attorney who has argued legal cases for minor parties and independent candidates challenging sore loser laws.
"There's precedents from just about every state that has a sore loser law that they don't apply to primaries," Winger said. "If I were Donald Trump and I knew I was going to be running outside the major presidential parties, I would not file for the Texas primary and South Dakota, and probably the Ohio one, although there's a flaw in the Ohio law."
Winger said the Ohio law specifically prohibits a sore-loser candidate from appearing as a minor candidate by petition, but minor party bids are typically nominations.
Brown said when he was doing research for Johnson's Libertarian campaign in 2012, he found five states that were actively enforcing sore loser laws. including Michigan and Mississippi. He argued an unsuccessful challenge to Michigan's law, which the court ultimately upheld in part because candidates can run as independents under the statute.
Between the failed presidential campaigns of Anderson, Johnson, Lyndon LaRouche and David Duke, most states have dealt with sore loser presidential candidates and the general election bids were allowed to continue, Winger said.
"When you add them all up, practically every state is on record saying our sore loser law doesn't apply to presidential primaries," Winger said.
Ultimately, a candidate with the resources at Trump's disposal would likely be able to mount a national third-party campaign, at least in terms of getting on the ballot, the experts acknowledged.
"If you're well-financed, it's hard, no doubt, but people have done it," Brown said. "Ross Perot did, John Anderson did it, Gary Johnson came real close to doing it recently. I think Trump could do it."
He continued: "Time, of course, is a problem, it is always is a problem. ... As a legal matter, Trump could wait until spring to make a decision. I'm not saying that's a good move strategically, but I don't think he has to make up his mind right now."