WASHINGTON — Joseph R. Biden Jr. on Tuesday pressed his case that he is the “most electable” Democratic presidential nominee, saying that while his primary rivals may draw large crowds in urban centers or excite young progressives, he retains the most racially diverse coalition of supporters.

The argument, bolstered by Mr. Biden’s strong poll numbers among black voters, accentuates the central thesis of his candidacy: that he is the Democrat most capable of beating President Trump.

It also comes as the Democratic field has winnowed in recent weeks, with some polls showing the race as increasingly a battle of three candidates — the more moderate Mr. Biden and the race’s two progressive New England senators, Bernie Sanders of Vermont and Elizabeth Warren of Massachusetts.

[Here’s the latest data on who’s leading the race to be the Democratic nominee.]

With that new landscape coming into focus, and with Mr. Biden facing scrutiny for a series of verbal slip-ups on the campaign trail, he and his team have leaned more into his support from black voters than ever. They have sought to cast their progressive rivals as representative of an elite class of Democrats that skews whiter, more educated and younger. They have also sought to counter critics who have tried to paint Mr. Biden’s candidacy as one-dimensionally focused on the white working class.