As the 2016 election finally kicks off in earnest, a cerebral and prominent New Englander with impeccable socialist credentials is challenging Hillary ClintonHillary Diane Rodham ClintonO’Malley tells Dems not to fear Trump FBI informant gathered years of evidence on Russian push for US nuclear fuel deals, including Uranium One, memos show Pelosi blasts California Republicans for supporting tax bill MORE from the left. In interviews and on social media, he has moved aggressively to connect Clinton with unpopular Wall Street names like Goldman Sachs. Despite his age and his thinning grey hair, he hasn't lost his fiery  progressive spirit or his legions of fans.

If this sounds like Sen. Bernie SandersBernard (Bernie) SandersDe Blasio headed to Iowa to speak at political fundraiser Yes, spills happen — but pipelines are still the safest way to move oil Why sexual harassment discussions include lawmakers talking about Bill Clinton’s past MORE (I-Vt.), think again. Ralph Nader is back.

No, Nader isn’t running for president this cycle. But Sanders is now reviving many of the same messages and tactics that Nader used when he ran an insurgent third party bid in 2000.   

Indeed, the similarities between Nader and Sanders are striking. Sixteen years ago, a nationwide progressive movement sprung to life determined to put Ralph Nader in the White House, no matter the obstacles and naysayers. Just like Bernie’s campaign today, Nader took up pure progressive proposals on critically important issues: a strong environmental platform, meaningful consumer protections, and far-reaching campaign finance reform. On a wide range of issues, from executive pay to financial regulation to environmental degradation, Nader sounded important alarm bells that society should have heeded. 

But Nader's electoral tactics were ultimately counterproductive. Despite his important message and excellent intentions, we're all paying the price for his campaign's blunder in 2000.  There's an important lesson for progressive primary voters flirting with a Sanders nomination today: We can't let the perfect be the enemy of the good. 

If not for Ralph Nader, George W. Bush would have been a rancher in Texas on 9/11. Say what you will about how events unfolded after Nader got his name on the Florida ballot. Sound off about Jeb Bush and hanging chads. Inveigh at the Rehnquist Supreme Court’s one-night-stand with judicial activism. But all of that happened after Nader took almost 100,000 votes off Gore.

We all know what happened after that: Two massive wars that have yet to end. An acceleration of greed and financial malfeasance, followed by the worst financial crisis since the Great Depression. Giveaways to big polluters and absolute failure to act in the face of climate change.  Bush slashed taxes on the richest of the rich and appointed 325 federal judges and two Supreme Court justices who will push our legal system to the right for a generation. It is hard to predict all of the unintended consequences that a Sanders loss to Sen. Ted CruzRafael (Ted) Edward CruzTexas Republicans slam White House over disaster relief request Dem rep: Trump disaster aid request is 'how you let America down again' Moore endorsements disappear from campaign website MORE (R-Texas) could entail, but they would likely be even worse.

While Sanders isn’t as much of a purist as Nader, they both tried to argue there is no difference between the Republican and Democratic parties. In an unsuccessful run for Vermont governor as an Independent in 1986, Sanders was vehement that "it is time to stop the tweedledee, tweedledum politics of the Republican and Democratic parties." During the 2000 election, Nader borrowed the same language calling Bush and Gore “Tweedledee and Tweedledum.”

Today, Sanders portrays Hillary Clinton as a Goldman Sachs lackey and part of a rigged establishment that includes both Republicans and Democrats. In 2000, Nader similarly labeled Gore ''an environmental imposter'' and even sent a letter to voters concerned about the environment slamming Al GoreAl GoreOvernight Energy: Trump officials defend fossil fuels, nuclear at UN climate summit | Dems commit to Paris goals | Ex-EPA lawyers slam 'sue and settle' policy Al Gore: A new president in 2020 could keep US in Paris agreement Ron Klain: ‘It’s not even going to be that close’ MORE’s green credentials and saying any environmentalist who supported Gore was adopting a "servile mentality."  

If this year’s voters are tempted by the same ideological purity that drew those 100,000 Floridians to Nader in 2000, the result could be eerily similar.   

Imagine the onslaught the right-wing machine will direct at Bernie if he wins the Democratic nomination. They will hammer him for only recently adding the word "democratic" in front of his long-time “socialist” moniker. They will bring up Sanders past affiliation with the Liberty Union party which advocated for nationalizing U.S. banks and industries. And then just wait for the evangelical right to start painting him as Godless for eschewing organized religion. No president and precious few members of Congress have ever been elected without embracing religion. As a fellow agnostic, I don’t think that is fair. But it is reality.

Sure, the hardcore Democratic base would be fired up with Bernie as the nominee.  But they were similarly fired-up for nominee George McGovern—who lost every state, save Massachusetts and the District of Columbia, in the 1972 race against Nixon.   

Ultimately, progressive Democratic voters shouldn't let the perfect be the enemy of the good. From a progressive perspective, Bernie is hardly "perfect" and Hillary isn't merely "good."   While Bernie attacks Hillary incessantly for being soft on big finance, Nader was similarly incessant 16 years ago in attacking Al Gore for being soft in his advocacy for the environment.  It's now hard to imagine the Nobel Peace Laureate and environmental trailblazer as a friend to polluters.  

Progressives may again decide to opt for ideological purity. If they do, President Cruz may just get his chance to one-up President Bush’s abysmal record.

Oliver was director of Communications between 2007-2010 for the Clinton Global Initiative, and was an advance associate for the White House during President Obama's 2012 re-election campaign.