Forget about the candidates. The toughest campaigner at next year's political conventions may be the National Journal, which will publish a free daily newspaper at each gathering in an effort to promote circulation.

"If we can produce something that people are going to read while on the convention floor," says National Journal publisher and president John Fox Sullivan, "we'll have 15,000 of the most important people in this country in politics and journalism--in effect, a captive audience."

The "National Journal Convention Daily," a tabloid with 24 to 40 pages, depending on advertising content, and a planned press run of 15,000 copies, will commandeer 15 of NJ's regular editorial staff of 20, supplemented by local journalism students in Dallas, site of the GOP convention, and San Francisco, where the Democrats will meet. In addition, Sullivan hopes to get stories from "top-notch political journalists" not subject to daily deadlines and to reprint articles from leading national papers not widely available at the conventions.

The plan arose this spring when Sullivan and ad director Roger Kranz were plotting how to take advantage of the election year to promote the influential journal of politics and government policy. They were discussing various special issues and features when "Roger got a somewhat grander idea," says Sullivan, who pitched the plan to both parties' national committees in June, stressing the paper's potential for communicating schedule changes, phone numbers and draft resolutions. Both quickly agreed; as did printers in Dallas and San Francisco, and NJ's owner, Government Research Corp.

The notion is not wholly new: During conventions, Congressional Quarterly traditionally distributes free daily research information to its clients on the floor. And in 1984, "we'll continue to put out timely background material whenever it seems warranted," says CQ publisher and editor-in-chief Wayne P. Kelley. "We wouldn't consider doing it in any other format."

But National Journal's plan is different. "The idea is to produce a newspaper which can be handed to delegates as they go into the hall" every afternoon, Sullivan says--which means closing each edition about 6 a.m. on Sunday through Thursday. "Of course, we know everybody there will have read four papers and seen the 'Today' show," he says, so the daily will concentrate on analysis of issues, profiles of delegates, officials and journalists, demographic statistics and other "scorecard" features. "A lot of it can be canned stuff done in advance," he says, and there will also be "some fun stuff": many cartoons, a political crossword puzzle, perhaps a set of trivia questions. NJ's editor-in-chief, Richard S. Frank, will edit the dailies from the convention cities while also overseeing production of the magazine back home. Difficult, but made easier because the two groups will be connected by computer telecommunications; "it has the potential of actually improving" the weekly, Sullivan says.

Not to mention the bank balance: The company expects to turn a profit by devoting 40 percent of the paper to advertising by corporate groups, credit cards, airlines and local merchants such as restaurants and limousine services. A full page will go for $7,500, and this week NJ is shipping 5,000 copies of a prototype issue to ad salesmen in San Francisco, Dallas and elsewhere.

But the bottom line is name recognition. Paid circulation at the 14-year-old weekly, 2,800 in 1976, has leveled off at the present 5,000 subscribers, who pony up $455 a year. "We haven't been out promoting it the way we should be," says Sullivan, who is shooting, somewhat uncertainly, for five digits: "Very few political-government magazines in this country are successful--it's just not that large an audience," despite an estimated nine or 10 readers for each copy of NJ. "I don't know what the market for this thing is. Ten thousand? Probably. But it certainly isn't twenty or twenty-five thousand."

NJ has budgeted $300,000 for the instant dailies, and if the tab "goes up by 10 or 20 percent," Sullivan says, "I'm not going to be upset." After all, "it's part of a whole range of things we're doing over the next political year, taking advantage of the occasion to get better known." Since early 1982, the magazine's gross revenues have doubled to $4 million as a result of two innovations: NJ began accepting advertising (unlike CQ, which fears that "our image of being nonpartisan might be hurt," says Kelley); and it bought "The Almanac of American Politics," the 1984 issue of which--containing a four-page plug for NJ--will be published next month with anticipated sales of 40,000 copies.

And this fall, Sullivan says, the magazine will start "to add to our coverage of the political world outside of Washington," beginning in late September with an original weekly column by Jack Germond and Jules Witcover. "It'll be a column for the pros," he says, "for John Sears, Pat Caddell, David Broder, corporate PAC givers--that sort of thing." Although an outside column could compromise NJ's reputation for stolid objectivity, Sullivan believes that " Germond and Witcover "are not seen as ideologically directed--they're skeptical of everybody." The column will continue through the election and "maybe beyond," Sullivan says. On Oct. 29, NJ's traditional triple-size election guide appears (mailed free to "Almanac" purchasers). And in January, the staff will start producing the National Journal Convention Daily as a four-page promotional mailer to news media and delegates.

With all this burgeoning commercialism, can NJ maintain its reputation for dispassion and high-minded credibility? Sullivan is optimistic. "The risk is not that we lose some money--but that we do not put out a product up to the kind of standards we like to think we have." Of course, "we may be basket cases by the end of it."