The race for the No. 1 spot may not decide the presidential election. But when Vice-President George Bush and challenger Geraldine A. Ferraro face each other on television tomorrow night, they will step into history without the benefit of comfortable rules to guide them.
Congresswoman Ferraro's task will be keeping the momentum going, following Democratic candidate Walter F. Mondale's highly praised face-off with President Reagan. But she will also be carrying the additional burden of being the first of her sex to be nominated.
''She's the first woman running for this office, and like it or not, that's what people look at,'' says Rep. Patricia Schroeder (D) of Colorado.
''I think it's obviously very important,'' says Representative Schroeder of the first-ever national debate involving a woman candidate.
For Vice-President Bush the event is no less challenging, since no man has ever been in his shoes before. He has already received warnings from letter-writers that if he hits too hard, he'll be viewed as a bully, but that if he strikes a gentlemanly pose he'll be seen as weak or patronizing.
''He really views Ms. Ferraro as the Democratic nominee'' and plans to ''treat her as such,'' without regard to gender, says Shirley Green, deputy press secretary for Bush.
Moreover, the debate could be a dry run for a vice-president who might want to run for the No. 1 spot in four years.
The two sides are casting the vice-presidential debate in very different lights. The Bush camp almost plays it down.
''I think everybody agrees with the premise that it's the top of the ticket which wins elections,'' says Bush press aide Green. ''Certainly that's the vice-president's whole effort: Keep the focus on the top.''
She adds that there would have been no ''clamor'' for a vice-presidential debate had it not been for the selection of a woman candidate.
Democrats, still elated from the Mondale success in the Oct. 7 debate with Mr. Reagan, are holding out high hopes for a Ferraro victory in the debate, which will be televised Thursday (9 o'clock Eastern time) from Philadelphia.
''I think she's going to be great,'' says Rep. Mary Rose Oakar (D) of Ohio, a vice-chair of the Mondale-Ferraro campaign. ''She's going to knock him out, just as Fritz (Mondale) did.''
On Capitol Hill members of both parties are offering advice to their House colleague and to Bush to ''be yourself'' and don't try to cram in too many facts and figures.
''There's a tendency to over-prep,'' says House minority leader Robert H. Michel (R) of Illinois in his admonition to the vice-president. ''With his long experience in the office and all the information that he's privy to on a daily basis, he should just be himself.''
That advice extends to being a ''little overly delicate'' when confronting a woman opponent, says the Republican House leader. ''You can't come down as hard on a woman as you do on a man,'' he says.
Representative Michel, in an interview, offered no assessment of the Ferraro debating skill. He said he hasn't heard her in a major debating roll on the House floor.
''She's got a gift for gab,'' he allows. ''She certainly isn't tongue-tied.''
Rep. Tony Coelho of California, chairman of the Democratic Congressional Campaign Committee, says of the vice-presidential debate, ''She has to make sure she doesn't come off brassy, and he that he doesn't come off attacking a woman.''
While Bush, former head of the Central Intelligence Agency, is the more experienced of the two, especially in foreign affairs, Democrats hold that their candidate is solid on the issues. ''She was on the (House) Budget Committee and knows enough figures and facts,'' says Representative Coelho.
''If I were her, I would try to fend off a lot of aides who're trying to fill her with facts,'' says Representative Schroeder. ''Just let her be Gerry, and she'll do very well.''
Democrats are also relishing an opportunity to pin some of the Republican positions on Bush. The vice-president has already indicated that he is uneasy with the abortion issue.
''He's got a very difficult role to play because he doesn't really believe in his party's platform,'' says Congresswoman Schroeder.
''It's much more of a gamble for Bush than for Gerry,'' says Coelho, since Bush will have to take stands that the right wing of the GOP ''will be monitoring very, very carefully.''
But will the debate also be risky for Ferraro and the future of women running for the highest national offices? ''I just don't happen to think one debate makes or breaks a candidate,'' says minority leader Michel.
''If Bush blows it, nobody will say, 'Well, it's all over for men,' '' says Representative Schroeder.