In its latest poll, the Washington Post reports that Bernie Sanders is now in a commanding lead nationally:
The poll underscores how quickly support for candidates can change in national polls on the strength of results from individual primaries and caucuses. The findings come on the day of a Democratic debate in Las Vegas that will include Bloomberg on the stage for the first time and represents a high-stakes gamble for all the candidates.
This has not always been true, but it’s been true for a long time. IIRC, Bill Clinton was the last person to build a lead slowly, back in 1992. Since then, you either win early or you go home.