I've seen this question asked in different ways on various fora, so here's Slate's explanation of what happens to unused campaign cash after a campaign is over:

Who Gets Gephardt's Cash? - Where war chests go when campaigns fold.

...after all creditors have been paid off, it's likely that Gephardt will still have a few million dollars to spare. He could save the money for his next campaign for public office, but that route seems unlikely; Gephardt has announced that he will not seek another term in Congress, and it's likely that his political career is now over. He could also disburse the money to the Democratic National Committee or a Democratic state party, as Al Gore did last July. Gore, who still has about $6.6 million left over from his failed presidential bid, donated $450,000 to the Tennessee Democratic Party, citing a "special relationship" with the state he once represented in the Senate.

Gephardt could also give the money to any charities he sees fit, provided he doesn't receive compensation from the recipient organizations until the donations have been expended. And, of course, he could refund the donations, although this is a rarity; the bureaucratic headache of sending out $50 checks to thousands of individual donors is usually considered more trouble than it's worth.

The campaign funds cannot be employed for personal use, such as buying a hot tub or paying for a kid's braces. But there's a small loophole for ex-candidates who still retain public office, as Gephardt does. According to FEC regulations, leftover campaign funds "may be used to defray any ordinary and necessary expenses incurred in connection with the recipient's duties as a holder of Federal office, if applicable." Those expenses include travel costs associated with "bona fide official responsibilities"—the FEC guidelines specifically mention fact-finding missions as copacetic, for example. Gephardt can also use the excess cash to defray "the costs of winding down" his congressional office after his term is up at year's end, though he's only permitted 6 months to use the money to close up shop.

Gephardt is also barred from simply transferring his war chest to another candidate. His campaign can make donations to other candidates, but only according to the rules that govern individual contributions—a maximum of $2,000. If Gephardt wants the eventual Democratic nominee to have access to his campaign funds, his best bet is to sign over the pot to the DNC.

So there you have it.