The two-party system suffocates independent challengers. I would know.

During a recent town hall in Columbus, Ohio, Sen. Bernie Sanders said the unthinkable. At least, you would have thought he did, judging by the response of several Democratic operatives. Sanders was deemed “extremely disgraceful” by Donna Brazile, vice chair of the Democratic National Committee, and “a political calculating fraud” by Brad Woodhouse, a former DNC communications director.

What was his crime? The old-fashioned Rooseveltian New Dealer had answered a question about why he is running as a Democrat, instead of as an independent, with typical candor: “In terms of media coverage, you had to run within the Democratic Party,” he observed, adding that he couldn’t raise money outside the major two-party process.

As one of the more successful third-party presidential candidates in recent U.S. history, I know firsthand the obstacles Sanders might have faced if he had run as an independent. The reality is that Sanders is right, and the backlash against him reflects all too well what two-party tyranny can do to a more-than-nominal third-party challenger. This is especially true of candidates like Sanders, who — despite advancing political views similar to the classic Democratic New Deal platform — now sits well to the left of the party’s corporatist, hawkish establishment.

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I chose to run on the Green Party line in the 2000 presidential election with a pretty clear idea of what I was in for. I had run a limited write-in campaign in New Hampshire in 1992 and had accepted the Green nomination in 1996.

My interest in moving politics past the two-party duopoly began long before I first ran for president in 1996. Historically, many major reform movements (abolition, women’s suffrage, labor) have come out of smaller parties that never won national elections, starting with the anti-slavery Liberty Party in 1840. Several different parties for women’s suffrage followed. Then came parties representing farmers’ struggles against railroads and banks, a movement that peaked in 1892 with the Populist Party. Labor parties — which fought for fair labor standards, the right to organize and progressive taxation — rose to prominence in the 20th century, along with the Socialist Party of America, formed in 1901. But when the Communist Party got on the national ballot after World War I, it drew widespread venom, and the two major parties began to raise barriers to ballot access and undertake other efforts to prevent these small parties from competing in elections. Admiring these reform movements and critical of the Democratic Party’s decay, I knew what it would mean to run as a third-party candidate.

Just appearing on the ballot is a challenge for independent candidates. While any Democrat or Republican who wins their party’s nomination is guaranteed a place on general-election ballots nationwide, smaller parties must, in many states, petition election officials to be listed. And that is a delicate process, easy for the major parties to disrupt. Their operatives have a number of tools at their disposal to knock third-party candidates off the ballot, render their campaigns broke, and harass and ostracize them.

In 2004, Democratic operatives were especially zealous in their efforts against my campaign. They hired private investigators to harass my campaign’s petition circulators in their homes in Ohio and Oregon and falsely threatened them with criminal prosecution for fake names that saboteurs had signed on their petitions, according to sworn affidavits from the workers and letters containing threats that were presented in court. Our petitions were also disqualified on arbitrary grounds: In Ohio, complaints submitted in court and to the office of the Secretary of State by groups of Democratic voters led officials there to invalidate our petitions. They disqualified hundreds of signatures on one list, for instance, because of a discrepancy involving the petition circulator’s signature. In Oregon, Democratic Secretary of State Bill Bradbury retroactively applied certain rules in a way that suddenly rendered our previously compliant petitions invalid.

Democrats and their allies (some later reimbursed by the DNC, according to both campaign finance reports and a party official in Maine who testified under oath) enlisted more than 90 lawyers from more than 50 law firms to file 29 complaints against my campaign in 18 states and with the Federal Election Commission for the express purpose of using the cost and delay of litigation to drain our resources. “We wanted to neutralize his campaign by forcing him to spend money and resources defending these things,” operative Toby Moffett told The Washington Post in 2004.

Democrats falsely accused my campaign of fraud in state after state. In Pennsylvania, they forced us off the ballot after challenging more than 30,000 signatures on spurious technical grounds. My running mate, Peter Camejo, and I were ordered to pay more than $81,000 in litigation costs the plaintiffs, a group of Democratic voters, said they incurred. In an effort to collect, their law firm, Reed Smith ,which the DNC also hired in that cycle, froze my personal accounts at several banks for eight years. A criminal prosecution by the state attorney general later revealed that Pennsylvania House Democrats had, illegally at taxpayer expense, prepared the complaints against our campaign, and several people were convicted of related felonies. A federal court in Pennsylvania ultimately struck down the state law used against me that had led to the order that I pay the litigation costs. But Reed Smith was still allowed to keep $34,000 it withdrew from my accounts, because state courts wouldn’t let me present evidence that could have permitted me to recover the money.

With the exception of this handful of felony convictions, most of the partisans who fought to keep me from running got away with it.

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Given another chance, I still wouldn’t run as a Democrat; I continue to disagree with the party’s platform and direction. Sanders is different, though: However he’s appeared on Vermont ballots in the past, he’s really a progressive Democrat. He has caucused with the party in Congress for decades, even if its corporatist core has abandoned his New Deal priorities. This is perhaps why he has been able to make it so remarkably far.

But as the backlash against his Ohio comments demonstrates, the party’s patience with Sanders is wearing thin. With today’s dominant Democrats favoring hawkish foreign policy and the entitlements of Wall Street, Sanders is seen as a Trojan horse. Cries of “get out,” already sounding in some Democratic quarters, will become increasingly fervid, notwithstanding Sanders’s years of support for Democratic causes and his pledge to endorse the Party’s eventual nominee.

By running as a Democrat, Sanders declined to become a complete political masochist, and he avoided exposing his campaign to immediate annihilation by partisan hacks. Because if he had run as an independent, he would have faced only one question daily in the media, as I did: “Do you see yourself as a spoiler?” The implication being, of course, that he had no chance of winning. His popular agenda would have been totally ignored by a horse-race-obsessed mass media, which would have latched on instead to a narrative in which Sanders was unfairly hurting Hillary Clinton’s chances against whichever Republican wound up with the other major-party nomination, as if any Democrat is automatically entitled to the votes of progressives.

Knowing that this is the fate of most independent candidates, as he put it simply in Ohio, Sanders made the right choice to campaign as a Democrat. Should he win the nomination, he will have no ballot-access obstacles to overcome in the fall. He gets to participate in televised primary debates, widely covered and commented on by the mainstream media. His scandal-free record and appealing message have resonated among younger Democratic and independent voters who are the future of progressive politics. A loyal base that believes he has a viable chance to win has allowed him to smash through the ritual of catering to fat-cat donors and super PACs to amass a highly credible campaign treasury.

Collecting nearly $150 million so far at an average donation of $27 is already a historic breakthrough for future honest candidates to emulate. In the longer run, proving that outsiders to cash-register politics can compete in the same manner may be one of the two most important legacies of the Sanders campaign.

The other is that Sanders has demonstrated the relative weakness of the corporate Democrats and their major loss of trust among the people, especially the young. “It’s sad and ironic how undemocratic the party has become,” says Bill Curry, a former White House counselor on domestic policy to President Bill Clinton and now a writer for Salon. He compares the party to “a closely held PAC used mostly to advance the careers of political insiders and the interests of corporate donors.”

I believe that should Clinton overcome Sanders and claim the Democratic nomination, the party will continue to be the champion of war and Wall Street, little changed by the primary competition. But perhaps after the comparative success of Sanders’s campaign, this state of affairs will invigorate more courageous candidates to follow his lead in challenging establishment, commercialized politics.