(CNN) With less than two months before the first votes of the 2020 presidential race, Joe Biden has re-emerged as the clear front-runner for the Democratic nomination.

That simple statement, which is borne out in national and early state polling, seemed unimaginable as recently as two months ago as the former vice president looked to be badly losing altitude on virtually every measure of viability -- from debate performances to fundraising to polling.

So, how did Biden turn it around? That answer isn't simple -- it never is! -- because it was no single factor. But among the reasons for Biden's resurgence are a) the struggles of Sen. Elizabeth Warren (D-Massachusetts) b) skepticism over the "Medicare for All" health care plan backed by Warren and Vermont Sen. Bernie Sanders and c) President Donald Trump's relentless focus on Biden and his son, Hunter.

Before we get anymore into why it happened, let's look at the unquestioned steadying of Biden's standing in the race.

In the Real Clear Politics national polling average , Biden has now opened up a double-digit lead over his nearest competitor -- Sanders. That's a remarkable change since early October when Warren had actually overtaken Biden in the RCP national average.