But the reason we’re likely to see a long race for the Democratic nomination is that each of the three candidates is having trouble convincing voters outside their base that they can win.

Only 38 percent of Democrats in Iowa and 36 percent in New Hampshire told CBS News they believed that Biden would probably beat Trump. The former vice president is seen as having performed unevenly in key debates. And, at age 77, he acts much older than the 73-year-old Trump.

As for Sanders, at age 78 and only three months past a heart attack, he also doesn’t present the most vigorous profile. More important, as the only candidate to openly declare himself a “democratic socialist,” his detractors are convinced he would have trouble winning over independent voters in November.

Pete Buttigieg, who served as an intelligence officer in Afghanistan before entering politics, has no trouble convincing people he is vigorous. But he has no governing experience beyond being mayor of Indiana’s fourth-largest city (population 102,000). In addition, nervous Democratic consultants worry that religious black and Hispanic voters might not support a gay candidate.