First off, Republicans will not ever "flee" to the Democrats. The rules in the MAGA party are very simple: You swear undying fealty to King Trump, and you will win your local elections or at least not get primaried by someone Trump likes better. You step out of line, and your career in politics and possibly life are over. This basically guarantees you your job, which is all 95% of politicians care about.
In fact, you are already seeing Democrats collaborating with Trump for this exact reason. Job security. Principles? Who gives a shit. Ask Fetterman all about it.
Second, the DNC has always been in a civil war with itself. Bernie Sanders is not new (mildly put). But right now that civil war is on a low-burner, because the only "progressive idea" everyone is championing right now is: "Let's have a system where people can vote for their next king...I mean "president". Remember when we did that? We should maybe do that again...somehow".
We have already regressed about 150 years in just 2 months of Trump. Just today Mike Johnson is musing about getting rid of federal judges, the pitiful mini-check on Trumps power. The Democratic party will either have to engage in good old streetfights and become a revolutionary party, or they will find themselves in concentration camps. And that is not hyperbole. When Hitler finally had free reign after dismantling the German constitution via bullshit, the first thing he did is put the socialists and communists in camps. He did that LONG before he ever moved against jews. Killing the opposition party is always step one.
"Last time" Trump demanded of his VP to declare the vote count invalid. When his VP wasn't about to do that, he had a mob storm capitol hill. Now, how likely do you think is the possibility JD Vance will would do what Mike Pence did? The only way for him to screw over his boss is if he thinks he will replace him. Which he might, and then everyone is going to wish Trump was still in charge.