Independent Evan McMullin’s Path to the White House Includes Strategy Not Used Since 1824

— -- Independent presidential candidate Evan McMullin’s longshot candidacy is dependent on attempting to block Donald Trump and Hillary Clinton from obtaining 270 electoral votes and sending the election to the House of Representatives, according to a McMullin campaign memo obtained by ABC News.

"Donald Trump cannot win. So the goal must be to keep Hillary Clinton from reaching 270 electoral votes and send the election to the House of Representatives,” according to the memo titled “How Evan McMullin can win and why it’s so important,” written by chief strategist Joel Searby.

“Once in the House, against the backdrop of Trump and Clinton’s deeply divisive positions and after a strong electoral college showing, we believe Evan’s unifying message will prevail,” Searby writes under the subhead, "So what’s the end game?”

It’s an apparent acknowledgment that a traditional campaign won’t work for the independent candidate’s uphill efforts. An election hasn’t been decided by the House of Representatives since 1824.

If no presidential candidate “receives a majority of Electoral votes, the House of Representatives elects the President from the 3 Presidential candidates who received the most Electoral votes,” according to the National Archives.

If they fail to "elect a President by Inauguration Day, the Vice-President Elect serves as acting President until the deadlock is resolved in the House."

That McMullin has to finish among the top three electoral vote-getters means he’ll be on the offensive not just against Clinton and Trump but also Green Party nominee Jill Stein and Libertarian nominee Gary Johnson. The memo hits Stein for “warmed-over magical-thinking socialism and weirdness,” and slams Johnson, a former Republican governor of New Mexico, as “astonishingly weak and dangerous positions on foreign affairs and religious liberty.”

But blocking both Clinton and Trump from reaching 270 electoral votes will require a perfect storm for McMullin. ABC News’ most recent battleground map shows solid and leaning Democratic states putting Clinton at 275 electoral votes just as a baseline.

So either McMullin or Trump will need to pick off at least one vulnerable Clinton state – like Pennsylvania or Virginia – but McMullin has already missed those ballot deadlines. Other options include Colorado, where McMullin is on the ballot, and Nevada, where he is gathering signatures.

From there, Trump and McMullin would need to block Clinton from winning virtually every tossup state. McMullin, 40, is on the ballot in Iowa, but Trump would likely be on his own in Ohio, North Carolina, Florida and New Hampshire, where ballot deadlines have already passed.

But perhaps the most daunting task would be McMullin’s need to win at least one state of his own in order to keep both candidates under 270. His best chance is Utah, but an independent candidate has not won an electoral vote since 1968.

The McMullin campaign says they are committed to winning electoral votes and the former CIA counterterrorism officer and former chief policy director of the House Republican conference is not just a protest candidate.

"It’s been 48 years since anyone other than the major party candidates won a state’s electoral votes,” Searby writes in the memo marked “confidential.” "It will be a signal that a new generation of American leadership has come.”

The memo also outlines the campaign’s efforts to get on ballots all over the country. They are on the ballot in Utah, Colorado, Iowa, and Louisiana, according to the campaign.

They are also actively gathering signatures in Minnesota, Idaho, Virginia and Wyoming, but they are also working on different strategies for other states. Such strategies include getting on minor parties with presidential ballot lines in states where they may be available like Minnesota, South Carolina, Oregon, Florida, New York, Hawaii and Delaware, among others.

They are also pursuing legal challenges in states where deadlines have passed and, despite the difficulty, Searby writes in the memo, they will pursue write-in campaigns in some states.

"Write-in candidacies, while challenging, are another option,” Searby notes. "In normal circumstances one might dismiss this” but, they argue, "it’s not complicated. We actually have faith in the American people’s ability to write. And given their other choices, we like our chances."

ABC News' John Kruzel contributed to this report.