WASHINGTON -- It's not too late: If Donald Trump loses the Republican presidential nomination, he says he might run for the White House anyway. And if Trump wins the GOP nomination, anti-Trump candidates from the party establishment are considering doing the same.

While it's an uphill climb and state deadlines loom, there are several ways a third candidate could mount a general-election bid consequential enough to demand attention. "'Trump' is easy to spell," notes Democratic strategist Tad Devine, suggesting the billionaire businessman could try a write-in campaign for the nation's highest office.

On the Democratic side, Hillary Clinton and Bernie Sanders have promised to support the party's nominee. But all three remaining Republican rivals have walked back commitments they signed last year to support the party's nominee. On Fox News Sunday, Trump refused to rule out an independent or third-party candidacy if he fails to win the GOP nomination. "I'm going to have to see how I was treated," he told Chris Wallace. "It's very simple."

In an election season that already has upended the expected, there are the ingredients that have fueled third-party bids in the past. They include a mercurial potential contender with money — Donald Trump, meet Ross Perot —  and the sort of ideological divide that prompted moderate congressman John Anderson to run when the GOP shifted right in 1980 to nominate conservative Ronald Reagan. The power of the parties has gotten weaker since then and the rules for getting on state ballots easier. 

"Minor parties have been working and working and working all these years and it's slowly paying off," says Richard Winger, publisher of Ballot Access News, a newsletter that just might see a surge in readership this year. "Nobody notices because it's little incremental changes, but it's adding up."

In a little-noticed ruling last month, for instance, a U.S. District Court struck down the provision in Georgia that required independent presidential candidates to submit petitions with signatures equal to 1% of eligible registered voters. In Green Party of Georgia v. Kemp, Judge Richard Story ruled that the state's ballot requirement, one of the toughest in the country, was too onerous. He set an interim standard of 7,500 signatures until the state legislature acts.

"There are two candidates in this race who could be a third-party candidate, and that's Trump and Bernie Sanders," says Republican strategist Ed Rollins, who for a time ran Perot's 1992 independent bid. "If Trump chose to go outside and spend his own money, he'd create chaos," albeit with a better chance of helping to elect Clinton than himself, Rollins says.

Neither Perot nor Anderson won the White House, of course, but they participated in debates and affected the cross-currents of the campaign. Some analysts say Ralph Nader's campaign in 2000 as the Green Party nominee may have pulled enough votes from Democrat Al Gore in Florida to tip the closely divided state -- and with it the presidency -- to Republican George W. Bush, although Nader disputes that.

And if Trump is nominated? 

"If Donald Trump gets nominated and continues to drive his negatives through the roof, conceivably a third-party candidate could become the de facto Republican nominee," says GOP pollster Whit Ayres. Speculation has centered on establishment leaders who have been outspoken in criticizing Trump, including 2012 nominee Mitt Romney and former 2016 contender Jeb Bush. 

Last week, The Texas Tribune reported there was no record of former governor Rick Perry voting in the state's primary, although he had endorsed Texas Sen. Ted Cruz. That drew notice because it would preserve Perry's eligibility to run as an independent in the fall. While Perry last year described Trump's candidacy as "a cancer on conservatism," a spokesman said the former presidential hopeful wasn't interested in making an independent bid. 

Voters increasingly are open to voting for someone other than the Democrat or the Republican. In the most recent USA TODAY/Suffolk University poll, taken in February, only a third of those surveyed said the two major parties did a good job of representing Americans' political views. A 54% majority said three or more parties were necessary.  

Running for president this year as a third candidate isn't easy. That doesn't mean it's impossible. Let's count the ways.

1. Mount an independent campaign.

That's what Perot did in 1992. The Texas billionaire said he was open to a draft movement in February, opened a phone bank in March and after an in-and-out campaign was on all 50 state ballots in November. There were times during the campaign when he led Republican George H.W. Bush and Democrat Bill Clinton in national polls, and he participated in the fall debates.

In the end, Perot carried 19% of the popular vote, the most for any third candidate since Teddy Roosevelt's bid in 1912 as the Progressive "Bull Moose" party's nominee.

Texas has the earliest deadline and some of the stiffest requirements for an independent presidential candidate to get on the ballot. By May 9 -- just five weeks away -- a candidate must submit petitions with 70,939 signatures. Realistically, the number has to be much higher as a cushion for signatures that might be thrown out as ineligible. North Carolina's deadline is next, on June 9. Three other states (Illinois, Indiana and New Mexico) have deadlines in late June.  

But thirty-one states and the District of Columbia don't have deadlines until August, and many require only a nominal number of signatures, such as 275 in Tennessee and 800 in New Jersey. In Colorado, a check for $1,000 gets a candidate on the ballot. The final deadlines (in Arizona, Kentucky, Mississippi and Rhode Island) are on Sept. 9. 

That said, 11 states have deadlines before the Republican convention opens in Cleveland on July 18, and other deadlines begin to follow in a crush. 

2. Be nominated by a third party.

The Libertarian Party already is on track to be on every state ballot this fall. Party chairman Nicholas Sarwark, 36, whose family owns an independent car dealership in Phoenix, says more than one of this year's presidential contenders have been in touch to express interest in the Libertarian nomination. He won't name names or even specify if the calls came on behalf of Republicans or Democrats, or both.

"There's a lot of frustration that candidates feel like they weren't able to get a hearing from the Republican or Democratic races, that there wasn't enough oxygen in the room," he says. He says his party could be open to nominating a newcomer.  "It would be up to that candidate to make the case that they were Libertarian or Libertarian enough." 

The party convention is in Orlando over Memorial Day weekend, with none of the delegates bound to a particular contender beforehand. The first nationally televised debate ever among Libertarian contenders was held last Friday on Fox Business News, featuring former New Mexico governor Gary Johnson, The Libertarian Republic founder Austin Petersen and anti-virus software developer John McAfee.

Johnson, the Libertarian nominee in 2012, doubts some high-profile Republican could move in and win the nomination.

"They're going to get their heads handed to them," he predicted in an interview, saying the GOP's conservative views on social issues would undermine their appeal among libertarians. That said, "they would bring a lot of welcome attention to the Libertarian Party" if they try.

Last month, a national Monmouth University Poll included a three-way race among Clinton, Trump and Johnson. The Libertarian candidate was supported by 11% even though three of four of those surveyed said they didn't know enough about him to have an opinion. 

Johnson's name "seems to be more of a placeholder for voters who are not particularly thrilled with either major-party choice right now," says poll director Patrick Murray. "There is an appetite there for a third-party candidate (but) it's still too soon to tell whether that will be a realistic option."

3. Cobble together different third-party lines.

Then there are all those other third parties that already are on some state ballots, but not all.

"People who think it is too late for Donald Trump to run after he is rejected by the Republican National Convention ought to realize there are ballot-qualified parties that would probably nominate Trump," Winger says. He listed 13 of them as prime prospects. They range from the Alaskan Independence Party (Sarah Palin's husband, Todd, was once a member) to the Independent American Party in Utah.

4. Run as a write-in.

Forget ballot access: What about running as a write-in?

Five states -- Hawaii, Louisiana, Nevada, Oklahoma and South Dakota -- have laws that ban all write-in votes, and South Carolina bans write-in votes for president. Election officials in Arkansas and New Mexico say they won't tally write-ins, and Mississippi law says the state will count them only if one of the candidates on the ballot has died.

Still, that leaves 42 states and the District of Columbia that do allow write-ins and will tally them. Most require a write-in candidate to submit a declaration of candidacy before the vote.   

It's a path that most realistic for someone with a fervent following and a famous name -- and, ideally, one that is easy to spell. That's not a frivolous concern when it comes to making sure a write-in vote counts: When Alaska Sen. Lisa Murkowski lost the Republican nomination in 2010 and then ran as a write-in, she aired a TV ad that showed a faux spelling-bee contestant sounding out her name, letter by letter. (Murkowski became the first senator in a half-century to win with a write-in campaign.) 

"If Trump doesn't get the nomination, I think he would organize a write-in campaign," predicts Rick Tyler, a former top campaign aide to Cruz, Trump's top rival. "In some ways, Donald Trump already is a third-party candidate. Right now, he is effectively a third-party candidate, running for the Republican nomination."