Joe Lockhart was White House press secretary from 1998-2000 in President Bill Clinton's administration. He co-hosts the podcast " Words Matter ." The opinions expressed in this commentary are his own. View more opinion at CNN.

(CNN) As a party loyalist, as opposed to someone supporting an individual candidate, the most important thing to me is nominating a candidate who can win in the general election. And a key component of that goal is making sure your party's nominee is fully vetted and scrutinized before they win the nomination, not after.

There are several reasons for this. First, you want to make sure the nominee can take a punch, because once the opposing party gets involved, you can be sure these will be delivered aggressively. Second, and as important, you want to fully air all controversial statements, votes and other activities before the general election, so that your nominee can legitimately claim these issues are old news and have been addressed. There is nothing more dangerous than having a new issue raised by your general election opponent at a key moment in the race.

Thus, while it may seem counter-intuitive, I want all the primary candidates to scrutinize their opponents, attack their records and have full coverage of it in the media. These scrutiny stories need three elements -- good opposition research, an enterprising reporter and another candidate willing to make the case in public for the relevance of the information.

In the race for the Democratic nomination, the process has worked fairly well. Opposition researchers, reporters and other candidates have made strong cases raising questions about the records of former Vice President Joe Biden, Sens. Kamala Harris and Elizabeth Warren, and former South Bend Mayor Pete Buttigieg. With differing degrees of success, each of these candidates has been forced to deal with their own political vulnerabilities. At a minimum, most of what President Donald Trump will attack them on has already been litigated in the primary run.

The exception to this rule, and a worry to many Democrats, has been the relatively lax scrutiny of Senator Bernie Sanders.

To be sure, Sanders' signature idea -- Medicare for All -- has been dissected and debated. And the staggering total cost and severe political liabilities of Sanders' full agenda is now beginning to be examined, as my CNN colleague Ronald Brownstein wrote recently.

However, there has been far too little discussion of Sanders' 50-year record, nor have we seen a real view of how he would defend that record against a Republican onslaught.

He's been lucky to avoid such an inquiry, because, for whatever reason, the media has discounted his ability to win the nomination. Press scrutiny moves on a sliding scale -- the closer you get to the nomination, the more intense the spotlight becomes. And opponents of Sanders have been reluctant to take him on given the intensity of support from his voters and their knowledge of how important those voters will be in the fall.

The result is that one of the leading candidates for the nomination has largely escaped the kind of scrutiny his opponents have been put through and that should worry every Democrat. His vulnerabilities, like the rest of the candidates, should be tested before Democrats gather in Milwaukee later this summer.

So, what might that look like? Though Trump would have an army of researchers digging into his past in courthouses and video archives, a few Google searches reveal a vast trove of controversial information that I'm confident is unknown to most Democratic primary voters. Even if none of these are themselves politically fatal, the totality would seem to offer the Trump campaign an enormous set of targets.

Let's start with what he calls himself (and the label Trump tries to stick to every Democrat): a socialist. There's no ducking this one -- Bernie has said a lot over the years, and a lot of it on camera. The Associated Press reports that in a 1983 reelection debate he said, "I am a socialist, of course, I am a socialist." The Washington Examiner reported last year that in 1980 and 1984, Sanders campaigned for the Socialist Workers Party. The report said that Sanders "proudly endorsed and supported" the Marxist group's presidential candidate, Andrew Pulley, in 1980. According to the report, in a press release, Sanders said, "I fully support the SWP's continued defense of the Cuban revolution."

Sanders' support for Fidel Castro came up during a 2016 presidential debate when Hillary Clinton questioned him about remarks he made in a 1984 video in which he said , "Everybody was totally convinced that Castro was the worst guy in the world...They forgot that he educated ...kids, gave them health care, totally transformed the society." Although Sanders agreed that Cuba was undemocratic and authoritarian, he seemed to refuse to take back his comments. The 1984 video and his refusal to disavow his comments could be a vulnerability for Sanders in 2020.

In the 1970's, Sanders called for the abolition of the CIA , reducing the military, and returning to local militias, and the Coast Guard, to defend the homeland. Once in Congress, he called for spending cuts of 50% in the Pentagon's budget.

Sanders has also made statements that will cause problems with key Democratic constituencies. The allegations of sexual harassment by members of his 2016 campaign team were covered (Sanders said he didn't know about the allegations during his campaign but apologized), but little attention was paid to an article he wrote in 1969 , exploring studies on stress and wondering if there could be a link between what parents taught their daughters about sexual mores and their child's likelihood of developing breast cancer.

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In 2011, he suggested several times that there would be value in a primary campaign against Barack Obama, one of the most revered figures in the Democratic Party. And in 2018, on the 50th anniversary of the assassination of Martin Luther King Jr., Sanders went to Jackson, Mississippi, and trashed the party and its first African American president. Speaking to a mostly black crowd, Sanders said , "The business model, if you like, of the Democratic Party for the last 15 years or so has been a failure ... People sometimes don't see that because there was a charismatic individual named Barack Obama. He was obviously an extraordinary candidate, brilliant guy."

It's well-known that Sanders has flipped his position on guns. He now says he supports gun safety, but in Congress, he initially voted against the Brady Bill, which requires prospective handgun buyers to wait up to five business days while a background check is conducted before completing a purchase, and later supported legislation immunizing the gun industry from several types of lawsuits.

Few are aware that before he got to Congress, he was even more radical. In 1972, he backed a political platform's call for the abolition of all laws that interfere with the constitutional right of citizens to bear arms.

I've seen this kind of thing before. To depress turnout, the Trump campaign will micro-target this information to the constituencies who would be most repelled by them. And Russian operatives won't need to talk to Julian Assange or English spies to get this material; they'll just need a dial-up modem.

Debates give us some insight into how Senator Sanders responds when he's on the defensive. But the issues go well beyond what he may have told Senator Warren in a private meeting. He has a long record, and just like all the other candidates, will need to defend it sooner or later. For Democrats, sooner is a must option as we try to rationally pick our strongest nominee to defeat Donald Trump.