This will be remembered as the week former senator Bill Bradley took the great leap forward in his successful underdog campaign for the Democratic presidential nomination. Unless, of course, it turns out to be the week Vice President Al Gore finally decided to strike back.

For Democratic Party professionals, it was a week of contemplating a national poll conducted by the Wall Street Journal and NBC News. The poll showed that while Texas Gov. George W. Bush led Gore by 17 points, he led Bradley by only 9.

One poll is no big deal, as Gore's lieutenants insist. But its message is very dangerous to Gore. That was underscored yesterday when Sen. Daniel Patrick Moynihan (D-N.Y.) endorsed Bradley and said: "Nothing is the matter with Gore, but he can't be elected."

Here's the key to understanding the Democratic nomination battle: For the majority of Democratic voters, neither candidate arouses intense loyalty or antipathy, so many Democrats will be more than usually swayed by their judgments of who is most electable.

Worse for Gore, polls show Bradley appealing to politically independent swing voters. That only increases the Bradley temptation. As one Gore supporter, who asked not to be named, mourned: "People are for Gore in a perfunctory way. There's not any commitment."

The Gore apparatus can point to a couple of good weeks on the campaign trail and genuine enthusiasm among some key supporters, California Gov. Gray Davis being the most important. The Gore camp also has reason to complain that national political commentary treats the vice president with about as much respect as the Russian economy.

If he wears a suit, he's a stiff guy in a suit. If he wears an open shirt, he's a stiff guy in a suit faking it. He gets no credit for Clinton's achievements, inherits all the baggage -- and finds his political skills compared unfavorably with Clinton's. To paraphrase an old Chicago political joke, if Gore walked on water, the headlines the next day would read: "Gore Can't Swim."

Unfair? Absolutely. But that's the way of presidential campaigns. Geoff Garin, a neutral Democratic pollster, cites two immutable rules of politics: "Nothing succeeds like success; and never miss a chance to kick a man when he's down."

Bradley's run so far is rated a success for tangible reasons. He's raised nearly as much money as Gore and spent it more prudently. He has a smart, lean political team enthusiastic about its man and his ideas. Gore has hired lots of able consultants -- his campaign might be called the Coalition of Democratic Consultants -- but as one political professional said: "You don't get a sense from Gore people that they are hungry. You look around at the Bradley people and they are hungry."

Comments such as that one are another part of Gore's problem. He's labeled the Democratic Establishment candidate, yet my interviews with members of that establishment suggest profound private doubts.

But the other reason Bradley is doing well could provide the opening Gore needs to counterattack. On the one hand, he is making inroads into the Democrats' liberal wing with his talk about universal health coverage, child poverty and his critique of the welfare bill Clinton and Gore supported. "He has sung the music of liberalism," says Robert Borosage, a leader of the party's liberal wing. "Liberals have been yearning for someone in the Democratic Party to respect them."

But Bradley is also gaining ground in independent and centrist constituencies. Is there a contradiction here for Gore to exploit? "Bradley is trying to argue it both ways," says Harrison Hickman, a neutral Democratic pollster. "Bradley's doing well among Democrats because he's more liberal. And he's doing well in the general public because he's more independent. . . . There's a certain amount of political jujitsu going on here."

Bradley could pull this off. He appeals to liberals with what he says and to independents with who he is -- a Mister Clean semi-outsider. But now he'll face a fight from Gore, whose campaign is preparing to engage Bradley on issues and to test him in debate, a Gore strength. Gore will be reluctant to split his party ideologically, so Bradley will be challenged first for inconsistencies in his record. You can also expect a challenge on Bradley's foreign policy views.

Gore does have one advantage. When former vice president Walter Mondale was upset in the 1984 New Hampshire primary by former senator Gary Hart's insurgency, Mondale only fully realized the danger he was in the weekend before the vote. Bradley is so strong that Gore has been given plenty of warning. Gore's fate hangs on how well he uses it.