Will House Democrats defend the speaker against unseating?

First of all, all our Constitution states is that the Speaker should be elected by a majority of the House. But normally the minority party votes for their candidate, so in order for the majority to win that Speaker election their entire Caucus would need to agree on the candidate, meaning they have to keep their caucus in line.

All the other noise around the election is in regards to the rules, which are set in each Congress. McCarthy was forced to agree to that ridiculous rule that a single member of the Majority could force a vote to "vacate the chair" in order to get enough support to get the job. If that motion passes, then the body can do nothing else other than hold Speaker elections. That single-member threshold is not normal.

So, all Democrats have to do is vote against that motion (or even abstain) so that the "no" votes outnumber the "yes" votes, and the motion fails and Johnson keeps his job. I think simply allowing the Ukraine vote earned Johnson this sort of protection. (McCarthy didn't earn this because he didn't keep up his end of deals with Democrats. Every Democrat voted for the motion to vacate, so the objectors only needed a handful of votes to get the motion to pass.)

But there is another thing to consider: the Speaker has broad discretion to set the agenda in Congress, but that is subject to a majority vote, too. Typically, these votes are just a formality, because the Speaker's party already has the majority and will support his agenda. But if there are enough defectors in the majority, these routine votes will fail, and the House would be at a standstill, unable to act on anything. If Johnson still needed help to get an agenda passed, that is where there would have to be more serious dealmaking. If Jeffries needs to help Johnson run the House, then Jeffries needs to get something out of it.

Well, the House already passed Ukraine aid, which is one of the things that Marge's FART squad was set up to prevent. So unless there's another big set of legislation coming down the pipe that Democrats really want out of Johnson, I don't see much use in them keeping him in the chair. There's only the tiniest, atom-thick difference between him and those "Freedom Caucus" chucklefucks, so it probably won't get worse for the Dems if another speaker election happens. One possible effect of another speaker election, however, (especially with all of the recent Republican "retirements") is that Jeffries could squeak into the speakership role. So, very little downside, some tiny chance at upside.

PSA - this is just my mostly uninformed opinion on the matter. Do not mistake anything said in the above paragraph for facts.