FOR THOSE who have worked in campaigns of such tottering financial standing that, when the paychecks were finally issued, the headquarters emptied as everyone dashed out to a bank that would convert them to cash, the question seems obvious: Why would anyone extend credit to a political campaign? Yet people do. They range from giant long-distance phone companies to owners of Holiday Inns in Iowa and New Hampshire, and the interesting thing is that most of them sooner or later get most or even all of their money back. For that they have mostly to thank the federal campaign finance laws. These require campaigns to keep their treasuries open, and they regulate the extent to which creditor businesses can write off debts owed by campaigns (so corporations won't in effect make after-the-fact contributions to the campaign).

The 1984 campaigns have yielded very different debt pictures. The tightly managed Reagan and Mondale campaigns have no significant debts, and neither does the anything-but-tightly-managed Jackson campaign. Sen. Alan Cranston has lowered his debt from $1.3 million to a mere $175,000 by patient phone solicitations, and has more than $1 million cash in the separate fund for his California reelection campaign next year. The big debtors now are Sens. Gary Hart, who owes $3.6 million, and John Glenn, who owes $2.7 million. Both used to owe more, however: $4.7 and $3.5 million, respectively.

How do you end up owing so much? Well, campaign finance is not an exact science. Campaign spending decisions have to be made quickly, with TV time-buying decisions for successive primaries coming up every day; and you can't be sure how much money is coming in. Presidential candidates are congenital optimists (why else would they take a chance and run?), and sometimes they gamble. Mr. Glenn stayed in two weeks past New Hampshire, until Super Tuesday; that cost him more than $1 million and gained him few votes. Mr. Hart, who had money pouring in so fast after New Hampshire that his staff had trouble opening the envelopes, saw it dry up after his defeats in New York and Pennsylvania a few weeks later; but he kept spending through the convention in San Francisco. Mr. Hart has ambitious and detailed plans to pay off the debt by mid-1986; Mr. Glenn hopes to do as well but doesn't have a target date.

If they do take a few years, it won't be a record. There are still debts left over from the 1980 campaigns. Hitherto unknown creditors have asserted claims against Edward Kennedy's campaign, and work on paying off some $600,000 of Jimmy Carter's campaign debt only began last fall, after the Federal Election Commission ruled it was legal for the Democratic National Committee to assume those obligations. That's not the record either. The DNC spent the better part of a decade trying to pay off the Humphrey and Kennedy campaign debts from 1968.

The Republicans -- more united, better organized -- don't have these problems. They banded together after 1980 to pay off everyone's debts, and pretty much did. That leaves them ready to begin, like most of the Democrats, to raise funds and run up debts for 1988.