The biggest problem that the Democratic Party is facing is the ideological struggle that is happening within the presidential primary. Some candidates, like Bernie Sanders and Elizabeth Warren, are promoting the more progressive agenda. While they're putting forward their plans for Medicare for all, there are candidates like Joe Biden and Pete Buttigieg, that aren't too keen on the idea.
The party has been grappling with this issue ever since 2016, when Bernie Sanders gave Hillary Clinton a stronger challenge than she expected. With the range of candidates on the stage now, the differences are more apparent than ever. You have Pete Buttigieg, who is rising in early state polls, proposing "Medicare for all who want it," and Bernie Sanders talking about how he "wrote the damn bill" for the expansive healthcare program. Many supporters of the democratic party are also dealing with this problem at the state and local level.
Candidates running for Congress are being primaried by more liberal candidates who want to echo Alexandria Ocasio-Cortez, and are supportive of the idea of Democratic Socialism. Candidates running for the House and Senate are moving further to the left to try and unseat more moderate members of Congress. The party has adopted more liberal positions, and so have the candidates, in order to appease the more left-leaning supporters.
The party isn't looking like it will smooth over this issue any time soon. The candidates themselves are united in defeating Donald Trump, but they'll have to unite to choose one candidate.