NEWTON, Iowa — Joseph R. Biden Jr. has built an early lead in the month since he entered the presidential campaign, confidently projecting himself as the Democratic front-runner and the candidate best positioned to defeat President Trump.

But beneath the surface of a seemingly placid race is a much more volatile contest, as a series of primaries-within-the-primary unfold along lines that reflect some of the most animating forces in the Trump era: race, gender, age and ideology.

With a historically large field of 23 candidates apparently now set, Senators Kamala Harris and Cory Booker, both African-American, are competing with Mr. Biden and other candidates for the support of black voters; Pete Buttigieg and Beto O’Rourke, who are both under 50, are vying for the mantle of generational change; Senator Elizabeth Warren is encroaching on Senator Bernie Sanders’s support from the party’s left wing; and six women are making the case that it is long past time for a female president.

As the first debates loom next month and candidates begin to test initial attacks on Mr. Biden, and one another, these miniature races will help clarify who might emerge as the most formidable alternative to the former vice president. As Mr. Trump demonstrated in 2016, initial assumptions about who primary voters will and will not support can prove foolhardy.