It is the class composition of the parties.
Ignore maps that show electoral results by state and look at county maps or maps of U.S. House districts. At this level of granularity, state borders disappear. There are no red states or blue states. Instead, there are blue urban cores floating in a sea of red. Even the exurbs and rural areas in blue states like California and New York tend to be overwhelmingly red and Republican.
This is not a difference between “city” and “country.” Hardly any Americans live or work on farms or ranches anymore. The big divide is within metro areas, between the blue downtowns and their inner-ring suburbs that are home to the American oligarchy and its children and retainers, and the red exurbs; outer-ring suburbs tend to be battlegrounds between the Democratic and Republican coalitions. This geographic concentration hurts the Democrats in the Senate and the Electoral College. At the same time, Democratic blue core cities in majority red states can often circumvent state governments by appealing directly to Congress and to the enforcement layers of the federal bureaucracy and judiciary, as well as to the media and corporate elites controlled by the national party.
The Democratic coalition is an hourglass, top-heavy and bottom-heavy with a narrow middle. In addition to hoovering up the votes of college-educated Americans, the Democrats are the party of the Big Rich—tech billionaires and CEOs, investment banking houses, and the managerial class that spans large corporate enterprises and aligned prestige federal agencies like the Justice Department and the national security agencies. This mostly white and Asian American group cannot win elections without the overwhelming support of Black Americans, and smaller majorities of Hispanic and Asian American voters, clustered in the downtowns and inner suburbs. The high cost of living in Democratic hub cities forces out the multiracial middle; the exceptions tend to be civil servants like police and first responders and teachers who can (sometimes) afford to live in or near their downtown jobs.
The social base of the Democrats is neither a few liberal billionaires nor the more numerous cohorts of high-school educated minority voters; it is the disproportionately white college-educated professionals and managers. These affluent but not rich overclass households dominate the Democratic Party and largely determine its messaging, not by virtue of campaign contributions or voting numbers, but because they very nearly monopolize the staffing of the institutions that support the party—K-12 schools and universities, city and state and federal bureaucracies, public sector unions, foundations, foundation-funded nonprofit organizations, and the mass media. By osmosis, professional and managerial values and material interests and fads and fashions permeate the Democratic Party and shape its agenda.
While the liberal Big Rich cluster in silver apartments and offices in trophy skyscrapers in the inner core of blue cities, the elites of the outer suburbs and exurbs tend to be made up of the Lesser Rich—millionaire car dealership owners, real estate agents, oil and gas drilling equipment company owners, and hair salon chain owners. This group of proprietors—the petty bourgeoisie, to use Marxist terminology, compared to the Democratic haute bourgeoisie and its professional allies—forms the social base of the Republican Party, despite efforts by Sens. Josh Hawley of Missouri, Marco Rubio of Florida, and others to rebrand the GOP as a working-class party.
Which is not to say that the social differences between the two parties aren’t important. There are far more business owners and fewer managers of huge multinational firms or banks in red areas than there are in the class-stratified, hierarchical Democratic urban cores. There are fewer rich and fewer poor. If the social structure of Democratic cores resembles an hourglass, the shape of the Republican exurbs and rural areas looks more like a diamond.