Some of Sen. Bernie Sanders' critics are recycling the claim that the veteran 78-year-old lawmaker hasn't been sufficiently vetted by the press and his political competitors.

Democrats critical of Sanders say he's so far been treated more as a curiosity than as a real threat in the 2020 race.

Cable news hosts, commentators, and other critics of Sanders have picked up that argument and the media is turning its spotlight on the Vermont lawmaker.

Sanders' political opponents are also stepping up their attacks on Sanders, who will likely be a prime target during Tuesday night's debate in South Carolina.

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As Sen. Bernie Sanders cements his status as the 2020 Democratic frontrunner, some of his critics are beginning to argue he hasn't been sufficiently vetted by the press and his political competitors.

Democrats critical of Sanders say the 78-year-old lawmaker has thus far been treated more as a curiosity than as a real threat to the party establishment.

"Bernie Sanders has gone from gadfly to GOAT nearly overnight," one Democratic strategist, who requested anonymity to speak candidly about Sanders, told Insider. "So much of the attention has been on how he's done it that most people haven't taken a look at his votes and views."

Rodell Mollineau, a Democratic strategist who's advising a super PAC supporting former Vice President Joe Biden, made a similar case. He argued Hillary Clinton's campaign treated Sanders as "a nuisance and not a competitor" in 2016 and that Sanders' 2020 opponents have until recently been reluctant to alienate his enthusiastic base.

"Sanders has benefited from the bigotry of low expectations," Mollineau told Insider. "No one ever truly took a look at the efficacy of his plans, whether or not the numbers added up, or what they would do to the country as we know it."

Mollineau argued that Sanders' new frontrunner status will force the media and other candidates to take a deeper dive on Sanders' long record and more controversial positions.

But Ian Russell, a former deputy executive director the Democratic Congressional Campaign Committee, said he's "skeptical" of the argument that Sanders hasn't been vetted and argued that Sanders is teflon when it comes to much of the his opponents' criticism.

"Senator Sanders has been a national figure for half a decade, and his unapologetic embrace of democratic socialism isn't exactly a secret," Russell told Insider. "I think we have to face another possibility: that the voters gravitating to him simply don't prioritize what would come up in traditional vetting as many in the media and political punditry do, and instead are looking to him as an authentic personality who will shake up a system that hasn't worked for them."

Media trains spotlight on Sanders

Cable news hosts, commentators, and other critics of Sanders have picked up the argument that the frontrunner hasn't been sufficiently vetted. And over the last several days, Sanders has been subjected to heightened scrutiny in the press.

On Saturday, as the senator closed in on a landslide victory in the Nevada caucus, MSNBC host Nicolle Wallace claimed he "hasn't been vetted by either the press or the other candidates."

Media outlets have begun resurfacing favorable comments Sanders has made about leftist governments, including decade-old remarks about former Nicaraguan leader Daniel Ortega and his recent refusal to label Venezuelan strongman Nicolas Maduro a dictator.

In recent days, Sanders has infuriated many Cuban-Americans by partially defending former Cuban dictator Fidel Castro in a 60 Minutes interview that aired Sunday night.

"We're very opposed to the authoritarian nature of Cuba but you know, when Fidel Castro came into office, you know what he did? He had a massive literacy program," Sanders said in the interview. "Is that a bad thing? Even though Fidel Castro did it?"

Democratic 2020 presidential candidates former Vice President Joe Biden listens to Senator Bernie Sanders as former South Bend Mayor Pete Buttigieg looks on in the seventh Democratic 2020 presidential debate at Drake University in Des Moines, Iowa, on January 14, 2020. Shannon Stapleton/Reuters

The remarks have provoked hand-wringing among Democrats who think Sanders' defense of Communist regimes will alienate the Cuban-American community and conservative Democrats.

Recent reporting that the Russian government is interfering in the 2020 election to help boost Sanders has prompted stories about his past comments about Russia and why President Vladimir Putin thinks he'd lose to President Donald Trump.

Last week, The Atlantic reported that Sanders seriously considered primarying President Barack Obama in the 2012 election — a claim Sanders' campaign denies.

Sanders also stirred controversy last week by announcing he won't release additional medical records following his September heart attack — a move that critics called Trumpian. New reporting emerged this week undermining his case for limiting what he makes public about his health.

On Monday, MSNBC's Wallace congratulated CNN anchor Jake Tapper for a discussion he hosted in 2016 about a controversial 1972 newspaper piece Sanders wrote in which he described rape fantasies to illustrate his progressive argument about internalized gender norms. Sanders renounced the piece in 2015.

Biden, Bloomberg, and Buttigieg go on the attack

Sanders' political opponents have simultaneously stepped up their attacks on the Vermont lawmaker.

Tim O'Brien, an adviser to billionaire Mike Bloomberg's 2020 campaign, argued that Sanders' health records, legislative history, and "statements about women" haven't been sufficiently vetted.

"Bernie Sanders has sort of stealthily run along thus far without being really vetted his legislative or career, personal life," he told CNN last Wednesday.

O'Brien added, "And I think that's also very Trumpy."

On Monday, the Bloomberg campaign unleashed a torrent of attacks on Sanders, including a series of "satirical" tweets falsely accusing Sanders of praising a host of dictators. The campaign deleted the tweets after they sparked backlash from reporters and critics across the spectrum.

During a Tuesday CNN interview, O'Brien brought up Sanders' controversial 1972 essay and other "loopy" arguments the candidate has made about sexual repression, perhaps previewing attacks Bloomberg will launch during Tuesday's night's debate.

Former Mayor Pete Buttigieg also escalated his attacks on Sanders, tweeting that the senator "doesn't understand" that the next president needs to be "extremely clear in standing against regimes that violate human rights abroad."

And former Vice President Joe Biden on Monday released a new ad based on The Atlantic's reporting about Sanders' alleged consideration of a 2012 presidential bid.

"When it comes to building on President Obama's legacy, Bernie Sanders just can't be trusted," the ad says.