Jeff Bell and Bill Bradley both are worried seriously about taxes and inflation.

It's not that either is that pinched. Bradley, a former New York Knicker-bockers basketball star, and Bell, a onetime California speechwriter, both can afford to dress well and eat at decent restaurants. And this year both are spending money at an unprecedented pace.

What's concerning them so is that the pocketbook issue clearly is uppermost in the minds of New Jersey voters this year, and both Bradley and Bell are running - against each other - for New Jersey's lone up-for-grabs U.S. Senate seat.

"We seem to have a single-issue electorate this year," says Bell, who started out with a broader approach but has been emphasizing economic issues since he saw how well they clicked. Bradley also views the economic issue as paramount: "Inflation," he says, "is the most important issue here."

Indeed, the New Jersey race isn't the only one in which the economic issue is dominating off-year election bids. Polls show that almost everywhere in the nation, taxpayers concern is re-emerging, and with a vengeance. In short, the pocketbook issue is back as number one in American politics.

A Gallup poll taken earlier this summer shows an overwhelming 60 percent of American voters now view the economic question as decisive - compared with only 10 percent for the next-most'critical issue, foreign policy. And candidates on the front line are reporting similar responses.

In Carrollton, Ga., GOP congressional candidate Newt Gingrich reports that concentrating on tax cuts and anti-inflation measures has proved "enormously effective" in a race for the seat now held by retiring Rep. John J. Flynt (D-Ga.).

And aides to Indiana Rep. Floyd J. Fithian say the economic issue, along with local concerns about the abortion controversy, is clearly at the top of the voters' list this year. "Inflation and taxes" is the response. That's what voters bring up most often.

Analysts say it isn't difficult to see what has driven the electorate to its present fever pitch. Both government and private figures show the pincers of inflation and higher taxes have made most voters feel they're in a bind. The recent surge of inflation merely has exacerbated the problem.

What makes New Jersey's Bradley-Bell race so interesting is that, for all the talk about the state's often-unconventional politics, the Senate campaign this year may prove the most dramatic test between the competing Republican and Democratic approaches to the pocketbook issue.

Bell is basing his campaign almost exclusively on the GOP's new-found economics - the party-backed Roth-Kemp tax cut plan (which would reduce marginal tax rates by 30 percent over the next three years), the Steiger bill for cutting capital gains taxes, and rollback in Social Security taxes.

In all, several dozen House and Senate candidates are hewing in some form to the Kemp-Roth and Steiger bills. Rep. Jack Kemp (R-N.Y.), who along with Sen. William V. Roth (R-Del.) is a chief sponsor of the tax cut measure, calls this fall's congressional elections "a referendum on" the Roth-Kemp bill.

At the same time, Bradley is backing what amounts to a traditional Democratic solution - $25 billion tax cut, even more generous than President Carter is recommending, enactment of new tax incentives to encourage moderation in wage and price demands, and more "sunset" legislation.

There's no doubt their emphasis is getting voters' attention. Stephen Salmore, director of the Eagleton Poll, one of the state's most respected voter surveys, says his own sampling shows the economic issue hits home hardest in almost all income and ethnic groups.

And an informal sampling at the annual Flemington Fair, at which both candidates put in campaign appearances last week, turned up near-unanimous agreement among voters that taxes and inflation were at the top of the list. Most still were undecided about which candidate provided the best solutions.

"We've just gotta do something about the taxes," said Frank McElroy, 36, a salesman and father of three from nearby Livingston, N.J., who says he's a registered Democrat but is still undecided. "I'll be damned if I know what the answer is, but it has to be done."

Joan Boor, 40, a clerical worker from nearby Ironia, complains that with inflation and taxes rising so rapidly, "It's getting harder and harder for the middle class to stay middle class." Boor voted for Bell in the GOP primary, but is thinking about switching to Bradley in November.

Right now, Bell still is regarded as the underdog - in part because of Bradley's advantage in name recognition. "Nobody can start off as a former basketball player on the New York Knicks and not be ahead in two-thirds of New Jersey," says one longtime observer of state politics.

But the gap is narrowing slowly. Bell won a surprise upset over longtime incumbent U.S. Sen. Clifford P. Case (R-N.J.) that surprised almost everyone around, and is gaining gradually as the weeks go on. His own analysis credits the tax cut plan with increasing his popularity.

The question is, will the complex Roth-Kemp approach, along with the Steiger plan for cutting capital gains taxes, which has been branded by Carter as "a millionaire's bill," really win the hearts and minds of traditional Democratic followers, such as blacks and blue collar workers?

Bell contends his reception among such groups has been "overwhelmingly good." He concedes that the Roth-Kemp theory may be complicated, but says "the voters understand that the present system isn't working and that the incentives here are missing. They agree that cutting taxes now makes sense."

But Bradley insists that his own exchanges with voters show the bulk of New Jersey taxpayers are put off by the Roth-Kemp notion that taxes can be cut drastically without trimming spending on services as well. "What they want is relief - not some new economic theory," he says.

If any state can serve as a battleground over the tax issue, new Jersey certainly qualifies as a contender. Property taxes here are relatively high. The state recently imposed a new income tax, which isn't bringing in as much in additional revenues as expected. And there's talk of higher gasoline taxes.

The Washington-based Advisory Commission on Intergovernmental Relations, which tracks state and locak fiscal burdrns, says New Jersey's tax bite ranks as eigth or ninth most burdensome in the nation in terms of the economic plight imposed on taxpayers. California was third - before Proposition 13.

Moreover, many New Jersey taxpayers - including blue collar families - heavily favor of the kind of tax cut measures Bell has been promoting, from a reduction in capital gains taxes to a tuition tax credit. And many older voters here also are concerned about taxes on profits from the sale of homes.

But despite the importance of the economic issue, there's not unanimous agreement on how deeply it will cut in November - either her or in other parts of the nation. For one thing, voters may find other things to like or dislike about a candidate - personality, or effectiveness as a speaker.

But also, GOP strategists fret that Democrats in some election districts are beginning to blunt the GOP tax cut issue simply by out-tax-cutting the tax cutters - proposing as-large or larger tax cuts than Republican candidates without the complex ideological arguments that the Roth-Kemp bill entails.

In the New Jersey race, for example, the Eagleton Institute's Salmore says the fact that Bradley already has come out for a tax cut may "effectively have diffused the issue" in the state. "Sure voters want their taxes cut," Salmore says, "but they don't care about the nuances of how it's done."

Salmore, for one, suggests Bell may run into snags on a much narrower economic problem: The nearby New York newspaper strike is so crowding the TV advertising time that Bell may not be able to get the exposure he needs in a traditionally heavily Democratic state to beat the better-known Bradley.

There are other factors: Bradley also is a far more agile campaigner. Bell, who did his last stint in politics as a Ronald Reagan researcher before venturing out on his own, seems visibly less at home shaking hands and talking with voters - a factor that could prove crucial if Salmore's analysis is correct.

Whether Salmore proves right, or Bell is correct in contending the Roth-Kemp issue will take hold, there's no doubt that the seeds for a new voter protest over the pocketbook issue are firmly implanted, and growing well both in New Jersey and across the nation.