First, nobody believed he would run. Once he did, very few imagined he would attack Fox News and the legacy of George W. Bush. Then this week, Donald Trump again floated what would be his most shocking move yet: bolting the GOP and challenging its nominee in November.

And to the Republican Party's likely chagrin, the window remains open for him to do so.

The billionaire winner of last week’s New Hampshire Republican primary suggested Monday in South Carolina he considers void an agreement he made with the Republican National Committee to support the party’s nominee. “As far as I’m concerned, they’re in default on their pledge,” he said.

If some turn of events does indeed prompt Trump to run outside the GOP's banner, he has heavy lifting ahead of him, though a bid wouldn't be impossible and money may lighten the load. The easiest path to the ballot – a major third party’s nomination – appears unlikely for Trump, forcing more legwork for a truly independent bid.

In most states, he likely would have to submit paperwork and a sizable number of petition signatures to earn a line on the general election ballot, with the first state’s deadline being Texas on May 9, says Richard Winger, an expert on the history of minor parties and laws that affect them. Four states have a June deadline.

North Carolina’s signature requirement for independent candidates – pegged at 2 percent of voter turnout in the last governor’s race, and due in June – is among the toughest, as is Oklahoma’s at 4 percent. California requires more signatures, but is well-stocked with professional canvassers.

If Trump were to run as an independent, “sore loser” laws also may cause a hiccup here and there, as such a law did in Michigan for former Gov. Gary Johnson of New Mexico, a 2012 Republican primary contestant who later became the Libertarian Party nominee.

But Winger, the editor of Ballot Access News, says most if not all “sore loser” laws are poorly written and that precedent across the country makes them unenforceable.

In addition to Johnson, who appeared on 48 general election state ballots, then-Rep. Ron Paul of Texas, a Republican candidate in 2008, appeared on Montana and Louisiana ballots after being nominated without his encouragement by minor parties.

1980 presidential candidate John Anderson was a Republican who lost several states before appearing as an independent on ballots across the country. His experience, in fact, may work as a double-edged sword against many "sore loser" and filing deadline laws.

Anderson, then an Illinois congressman, won federal court orders against early deadlines in Maine and Maryland. He also took Ohio’s independent candidate deadline to the Supreme Court and won on First and 14th Amendment grounds in Anderson v. Celebrezze.

In the 1983 ruling, the Supreme Court majority noted “several important third-party candidacies in American history were launched after the two major parties staked out their positions and selected their nominees at national conventions during the summer,” and that “the March filing deadline places a particular burden on an identifiable segment of Ohio's independent-minded voters.” About two-thirds of states then had deadlines in August or September.

June deadlines in five states since have been struck down by courts, Winger says, opening the door to legal action against Texas' May deadline and the remaining four June states.

If Trump turns in enough signatures and a state tries to block him from the ballot by pointing to a "sore loser" law, there's a conceivable way he could sidestep the ban, Winger says: by putting his son, Donald Trump Jr., on ballots where necessary, with Junior's electors vowing to vote en masse for Trump the senior.

“There's no power in Earth that could stop him," Winger says. To that end, laws in Montana and North Carolina intended to disqualify faithless electors "are toothless" if the entire slate switches over, he says.

So why could Trump not just snatch the endorsement of one of the major third parties, enormously reducing the effort required?

Members of the Libertarian Party, which has ballot access in more than half of states, probably would turn up their noses to the anti-interventionist who made the free market work for himself, says Nicholas Sarwark, chairman of the party's national committee.

“If Mr. Trump joined the Libertarian Party, he would be eligible to be nominated at our national convention,” Sarwark says. “It would be up to the national party delegates whether they wish to nominate a candidate that has taken a number of positions in direct conflict with the Libertarian Party platform, such as his support of using eminent domain for private gain.”

If Trump wants the Libertarian line, he should book a presidential suite in Orlando, Florida, for the convention during Memorial Day weekend in May. But, Sarwark adds: “My sense is that if he were to seek our nomination, he would suffer a huge defeat.”

The progressive Green Party, which has ballot access in nearly half of states plus an affiliate in the nation’s capital, also would be an odd fit for Trump, and it’s already shown a disinclination to celebrity, passing over actress Roseanne Barr last cycle in favor of lesser-known activist Jill Stein, who’s running again.

“It's technically possible for Donald Trump to seek the Green Party's nomination in the same way that it would be technically possible for Bernie Sanders to run as a Republican,” jokes Scott McLarty, media coordinator for the party.

“The likely reaction within the Green Party would be gales of laughter and, ‘Get lost,’” he says, though Trump is welcome to make McLarty eat his words at the party’s early August convention in Houston.

Another third party, the Constitution Party, generally is considered socially conservative. It’s unclear if it would back Trump, who once appeared on the cover of Playboy magazine but now says his favorite book is the Bible. The party chairman did not respond to an inquiry.

Winger, a Libertarian Party member, says he’s not much of a Trump fan, but if he were on the Trump payroll he’d suggest seeking out the nomination of 12 single-state parties with ballot access, many of whom have a centrist leaning.

The once-prominent Reform Party of the 1990s, whose nomination Trump briefly sought in 2000, has limited ballot access but could provide a small boost. Reform Party leaders did not respond to requests for comment.