Lots of different legal options but also lots of games are played. You can redirect it to a charity or to a state or national party campaign. One of the more common games which is legal is to declare your candidacy for some future office and hold it for that. Thus Bernie used his leftover Senate race funds to start in on his presidential campaign. The astute ones will recall that Beto was raising funds for his senate race hugely right up to the very last week of his campaign with no outlet for spending it. Then after he lost, he declared for president and then suddenly had all sorts of cash to just float around and be a free spirit, because he is legally allowed to transfer it to another campaign where he is the candidate, unless the donor specified that it was for just the previous campaign. Gets complicated too. You get a lot of big donors who donate large amounts but just for the candidate's general election campaign. And primary campaigns and general election campaigns are considered to be two different campaigns. So there are instances where if you specified that it is for the general campaign but the candidate did not get the nomination then- horrors of horrors- you have return it. There is also a whole nuther level of complication if you donated to a candidate by donating to one of their PACS. Different rules, different games with PACS. Like dat. and then more. And then you have John Edwards type games where you argue that the donor was giving to you personally rather than to your campaign so the money is yours. Not a trick you want to try at home. Also not to forget that we may assume that someone like Buttigieg will be dropping out by the next debate due to low polling but the opposite can be true too- ie. he has the cash to stay in and build name recognition right up and through the convention even if he drops in the polls. He sort of has a job but some of them don't. Castro, Beto, and a few more. Nothing else to do. Gillibrand and Klobuchar have/had staffs etc and needed/need to go back to work so they can't just hang around. Buttiplug probably wants to hold on to be a player at the convention - and broker his supporters and remaining cash for a future job.
Trump fundraising total during same time period was 125 million dollars----a presidential race record.