Some perfectly fine Democrats have been defeated in primaries by Lefties. These are loyal, cobalt blues, nothing purplish about them, in Congressional Districts safe from challenge by Grand Old Phascists, in entire states where the Democrat would be favored to win the Senate seat. In some cases these Democrats have been incumbents. They have proven their nous time and time again, including in Trump's two impeachments in the House. They wore their impeachment votes as badges of honor. Still they were primaried from the Left, in some cases lost. Why would that be? Let's hear from an incumbent Dem.:
"Members now realize that it's [impeachment] not that big of a deal. It's old hat...Especially when you can't deliver."
You don't get an "A" for effort any more; "Resistance" doesn't cut it any more. You need to win. After Trump's second presidential win, some incumbent Resistance Democrats faced angry constituents who demanded "blood in the streets", even their blood when they went back to their districts. The impeachment incumbents were shocked.
What this means is that many American voters of both major parties have lost faith in democratic theory, where the democratic process legitimizes the outcome; in fact, it is the only legitimizing process. Republicans from 2016 to present have totally abandoned democratic process for a focus on results. Small d democratic theory is that you can't win every time, you have to compromise, and you have to accept the results of free and fair votes. As Hubert Humphrey once stated, "Dukakis thinks as long as the pipes are shiny it doesn't matter if shit comes out". That was an American-context rejection of democratic theory. Sheepish political scientists would point to broader examples, such as the elections in Germany in the 1930's. No, the Nazi Party never won a majority of seats in the Reichstag, but like in Germany at the time, like in Israel throughout its history, close counts in horseshoes, hand grenades and parliamentary democracies. The theory in parliamentary democracies is that pluralities force compromise and you get a muddier, but better, representation of the whole electorate. So, although political scientists don't like to mention it, Adolph Hitler and Benjamin Netanyahu were the results that proved that democracies work. You can see why poli sci's don't like to talk about that.
Coming back to America, we have a quasi first-past-the-poll representative democracy. At the presidential level you do have to win a majority of electoral votes, but to accommodate the smaller, slave-holding states, you can win the electoral college and the presidency without winning a majority of the popular vote. Voila! Donald Trump. The pipes were shiny in 2016 and 2024. We still got shit. And, like in Germany and present-day Israel, shiny democratic pipes may result in the death of Democracy in America.
When "you can't deliver", the focus becomes on the process of democracy as the culprit. Donald Trump would not have accepted the legitimizing democratic process in 2016 had he lost the Electoral College (he did lose the popular vote); Trump, and a majority of congressional Republicans, did not accept the democratic process in 2020; they would not have accepted it in 2024; they might not except it in 2026. To him and them, only the result mattered.
When one side doesn't accept democratic process and the other side does, you have an unfair fight. That is what DSA, the broader Left movement in America today, and increasing numbers of Democratic voters realize. DSA wants to abolish the Electoral College, Democrats of all shades are fed up with the unfair fight, Resistance isn't enough, "especially when you can't deliver."