As former Vice President Joe Biden prepares to formally launch his 2020 presidential campaign, the central question he faces is whether his consistent leads in all national and most early state polls will withstand the scrutiny of a long presidential campaign. If Rudy Giuliani's campaign for the 2008 Republican nomination is any precedent, it could be a long time before that question gets answered.

Right now, though Biden is the polling leader, he's seen by many political observers, including myself, as very vulnerable, and he is not being treated as the clear front-runner. The thinking is that right now, he's polling well because of high name recognition and he's still enjoying the wave of popularity among Democrats from being associated with former President Barack Obama and for the gracious way he handled the public grieving after his son's tragic death. Once he becomes a candidate, the thinking goes, his vulnerabilities will come into focus: his age, his propensity for gaffes, his past stances that seem to be out of touch with the younger and more diverse Democratic electorate. At the same time, voters will be getting to know other candidates better, and his name recognition advantage will erode.

So far, Biden survived the first blast of scrutiny over his unwelcome contact with women, without much discernible impact on his polling. That suggests a reservoir of good will for Biden among a swathe of Democratic voters who aren't the same as those who dominate the discussion on Twitter. However, it may be a long time before Biden is really put to the test.

That's where the Giuliani precedent comes in. Many people remember that Giuliani ran for president in 2008, was the national front-runner at first, and then his campaign collapsed as he focused on winning Florida while not doing enough to win in early primary states. But what is often overlooked is just how long Giuliani spent as the leader in national polling.

Giuliani actually led in national polls throughout 2007 leading up to the start of the primaries. It wasn't until January 2008 that he surrendered the lead in the wake of Iowa.

Now, there are, to be sure, several significant differences between the two candidates and the two races. While Biden may have taken positions that are objectionable to the resurgent left-wing of the party, he does not have as many failed litmus tests that Giuliani did on abortion, guns, immigration, and a number of other issues. Biden also has significantly more national experience, less personal baggage, and is also stronger in early primary states than Giuliani had been.

On the flip side, Giuliani faced weaker opponents than Biden. In 2008, each of the other leading Republican candidates were unacceptable to a portion of the party — John McCain was hated by many conservatives, Mitt Romney was mistrusted due to his recent reversals on a host of issues, Mike Huckabee scared off non-evangelicals, and Fred Thompson, who carried the promise of uniting all factions, fizzled fast. In contrast, Biden faces a large and diverse field of candidates who are uniting around similar policy goals. The bottom line is that, for the most part, the major Democratic candidates would be acceptable to most Democratic voters.

Though no two primaries are the same, however, the reason I bring up the Giuliani example is that it's a reminder that a candidate who will ultimately lose can remain on top for a long time right up until voting begins. So it's possible that Biden remains the polling leader for quite awhile, generating a lot of stories about how resilient his candidacy is in the face of attacks, only to surrender his lead once voters in early primary states start paying closer attention to his rivals.

