WEBSTER CITY, Iowa — Iowa’s presidential caucuses disenfranchise huge blocs of voters. The state is 91 percent white. It is not easy to get to, or get around in. But to a greater degree than in recent campaigns, this unrepresentative and idiosyncratic state is proving that it is the only electoral battleground that matters for Democrats from now until caucus night on Feb. 3.

Former Vice President Joseph R. Biden Jr.’s campaign said it would have 110 staff members in the state by the end of this month. Senator Kamala Harris has promised to visit every week in October. Mayor Pete Buttigieg on Sunday kicked off a John McCain-like, everything-on-the-record bus tour and is advertising on local television and radio. Senator Amy Klobuchar is 49 counties into her tour of all 99 in the state. And on Monday, Senator Bernie Sanders will begin a “Bernie Beats Trump’’ tour of eastern Iowa to highlight what he says is his strength as a general election candidate.

As a new poll suggested a significant shift in the primary race’s top tier — with Senator Elizabeth Warren overtaking Mr. Biden for first place — candidates at the annual Polk County Steak Fry in Des Moines on Saturday tried to cut through the furor surrounding President Trump, Ukraine and Mr. Biden to deliver their message to voters still sifting through their preferences from one of the largest fields in history.