With Donald Trump the presumptive nominee of the Republican party, conservatives face their biggest crisis in generations. Professional Republicans are mostly boarding the “Trump Train," convinced their self-interest requires party unity, but principled conservatives find the choice between the dissolute, erratic Trump and the liberal, corrupt Clinton unacceptable. What comes next for them?

This insurgency should field a candidate for the White House in 2016, which would require a leader to offer him or herself to the voters on the November ballot. Trump and Clinton are the most unpopular nominees in the history of polling, so why can't the people elect a fresh face or a trusted elder statesman over these unsavory characters? Moreover, the danger for down-ballot carnage with Trump as the nominee is substantial, and an independent may provide electoral cover for vulnerable conservative officeholders.

Yet the calling for an independent candidacy is higher than this, for the ideals of conservatism are at stake. Trump is not like Barry Goldwater or George McGovern, decent men who were out of step with their times. They may have been unelectable, but at least each was bound by commitment to his principles. Trump has no principles and is quite pleased for us to know this. As he told California Republicans a few weeks ago, "Folks, I'm a conservative, but at this point, who cares? We got to straighten out the country." Real conservatives believe that it is their ideas that will "straighten out the country," but for Trump these were empty talking points offered to secure the nomination.

By selecting such a character, the Republican party has abandoned its commitment to conservatism. This is a dangerous development. Parties in the United States are meant to be more than just machines to further ambitious politicians, they are supposed to promote broad principles for the general welfare. The GOP was, for generations, the vehicle for advancing conservative ideology. This year it won't be.

Some may comfort themselves that Trump is sui generis, a one-off never to happen again. But why assume that? If the Republican party will abandon its principles for a failed casino mogul best known for tabloid antics and a reality TV show, what won't it abandon them for?

Conservatives cannot let this pass without a response, which is why an independent candidacy is essential. It isn't just a way to win the White House or hold the line in Congress, it is a statement of purpose and a warning to the Republican party that commitment to conservative ideas comes before party loyalty. If the party abandons the principles that have guided it for decades, it may no longer count on conservative support.

In this way, a conservative third-party candidate may serve the same purpose that Teddy Roosevelt's Bull Moosers did in 1912. Conservatives are inclined to castigate this party for its progressivism, and rightly so, but the Bull Moosers were reacting to a sense that the Republican party put special interests ahead of the general welfare. The first plank of the platform read:

Political parties exist to secure responsible government and to execute the will of the people. From these great tasks both of the old parties have turned aside. Instead of instruments to promote the general welfare, they have become the tools of corrupt interests which use them impartially to serve their selfish purposes. Behind the ostensible government sits enthroned an invisible government owing no allegiance and acknowledging no responsibility to the people. To destroy this invisible government, to dissolve the unholy alliance between corrupt business and corrupt politics is the first task of the statesmanship of the day.

This is a fitting description of the Republican party in 2016: the unprincipled Trump standing at the head of a corrupt political force, flanked on one side by K Street lobbyists who use government to line their pockets and on the other by professional politicians regurgitating Reaganesque talking points but lacking true beliefs. Having betrayed the principles that have animated it for so long, this party cannot secure responsible government anymore.

Here's how the Bull Moosers reacted to Republican corruption a century ago:

Unhampered by tradition, uncorrupted by power, undismayed by the magnitude of the task, the new party offers itself as the instrument of the people to sweep away old abuses, to build a new and nobler commonwealth.

Whatever one may say about the progressives, they weren't wrong about the GOP, which had been overrun by corruption. Instead of sitting idly by, the Bull Moosers rebuked their crooked old party and formed a new coalition.

Though Roosevelt did not win in 1912, the progressives eventually carried the day. They were such an electoral force that the Democrats, under Woodrow Wilson, gave them everything they could have expected. In 1916, the duly chastised Republicans brought the Bull Moosers back into the fold by nominating the progressive Charles Evans Hughes.

The 1912 election should focus conservatives on the bigger picture. This is not simply about one election, but about the ideas that generations of conservatives have advocated. If ever conservatives hope to win, they need to fight on behalf of those principles at every opportunity, not surrender for the sake of "unity" to a party that has disavowed their core beliefs.

It's time for a true conservative leader to come forward as an independent candidate for president—and time for conservatives to do everything they can on that leader's behalf.

Jay Cost is a staff writer at The Weekly Standard and the author of A Republic No More: Big Government and the Rise of American Political Corruption .