To be entirely fair, Democrats did not lose the House. They lost seats in the House at a time when they were expected to make gains, and they'll be much more vulnerable in the 2022 midterms.
There was a good piece in the Times by Thomas Edsall highlighting a conflict of interests for Democrats in that what can work with the presidential race doesn't necessarily help in the median congressional district.
https://www.nytimes.com/2020/11/18/o...il&login=email
Analysts and insiders are already talking *— sometimes in apocalyptic terms — about how hard it will be for Joe Biden to hold together the coalition that elected him as the 46th president. But it’s important to remember that conflicts are inherent in a party that seeks to represent constituencies running the gamut from Alexandria Ocasio-Cortez’s 14th district in New York (50 percent Hispanic, 22 percent non-Hispanic white, 18 percent Asian, 8 percent Black) to 7th generation Utahan Ben McAdams’s 4th District in Utah (74 percent white, 1 percent Black, 3 percent Asian, 17 percent Hispanic.)
Jonathan Rodden, a political scientist at Stanford who has explored the structural difficulties facing Democrats in his book “Why Cities Lose,” wrote in an email that the concentration of liberals in urban communities creates a built-in conflict for the party:
“The ‘presidential wing’ of the party,” he said, referring to the wing most concerned with winning the national election for the White House, “faces no incentives to worry about the geography of Congressional or state legislative districts at all.” The ideal platform for winning presidential elections, Rodden continued,Race, Rodden added,
might be one that hurts the party in pivotal Congressional races. This dynamic might be even more pronounced if the ”presidential wing” decides to pursue a strategy of mobilizing the urban base in order to win those pivotal states.
only enhances this effect. Given the urban concentration of African- Americans in Northern cities, the Democratic political strategy that maximizes the probability of winning the electoral votes of a pivotal state is probably more responsive to the policy priorities of African-American voters than the strategy that would maximize the probability of winning the mostly white pivotal exurban district in that state.