Tune in a Bruins or Celtics game on Boston television these nights and you are likely to see an ad in which the speaker says: "I am the only candidate for governor of New Hampshire pledged to veto an income tax and sales tax bill and to cut your utility bill."

No, kiddies, it is not Republican Gov. Meldrim Thompson, Jr., who has won three terms by taking "the pledge" to stop any broad-based tax measure.

It is his Democratic opponent, former state representative Hugh Gallen, who is going the governor one better by turning the tax issue against the man who has made it the cornerstone of his political career.

It will surprise no one if Thomson, a leading figure in the national conservative movements, wins an unprecedented fourth term next Tuesday.

What is surprising is that Gallen, a 54-year-old auto dealer from Laconia who tried twice for the Democratic nomination before winning it this year, has found a way to outflank Thomson on his own issue. Because of that, some Democrats and some pollsters say he has a chance of an upset.

The reason is CWIP - an acronym for te utility bill surcharge for "construction work in progress" on the controversial nuclear power plant in Seabrook.

Polls show most New Hampshire residents favo the Seabrook facility, which has been the target of unending environmental lawsuits and demonstrations. But they don't like the way they are paying for it.

The Public Service Co. of New Hampshire, principal developer of Seabrook, won permission from the public, utility commission to add a construction surcharge to its bills for the next four years, until the project begins producing power.

Democrats pushed an anti-CWIP bill through the legislature last June and Thomson vetoed it - handing Gallen his issue.

It is a live one, CWIP itself adds almost 10 percent to utility bills, and a higher fuel-adjustment charge which went in at the same time boosts the total surcharge to more than 17 percent. People are howling, and Thomson for once is on the defense on a pocketbook issue.

In a radio interview in Salem the other day, he insisted that "I have not changed my position one bit" because of the uproar. "The surcharge was necessary to produce the cash flow to induce the bond market to underwrite the $2.3 billion construction cost. There are 2,000 people working in new jobs at Seabrook now, contributing to the economy of this state. That power source is absolutely necessary to our future - to keep the plants we have here and to attract new plants.That is our lifeline to our economic future."

But Thomson has given a little ground and has won a promise from the utility that CWIP will increase no more than the cost of living, in the next three years and will phase out as electric production begins.

Gallen scoffs at such promises and says the surcharge was unnecessary in the first place and would be illegal in more than two-thirds of the states. "It's the first time Mel Thomson has imposed a tax on the average family in this state, and they're not going to forget it," he says.

Thomson has been immune to criticism on a wide variety of other issues just because he has enjoyed a reputation as a fierce opponent of taxes. In the past, Democrats have attacked him for starving social services and for embarrassing the state by his forays into international conservative causes from Taiwan to South Africa.

At a Rotary luncheon in Salem, a guest listened to Thomson's standard recital of the virtues of low-tax New Hampshire and then told him tartly, "I wish the elderly and the handicapped in this state were treated as well as the highways."

But Thomson has a good story to tell and the businessmen applauded it warmly.

"Because of the health of our economic climate" he said. "We have attracted 250 new industries in the last 5 years and existing industries have added 60,000 jobs. We have the lowest unemployment rate in New England - 3 percent - and we are the only New England state with a surplus in its employment security fund."

New Hampshire derives most of its revenues from a business profits tax and the profits of a state liquor monoply, heavily patronized for its low prices by residents of neighboring states.

But Democrats publicized a poll by White House opinion sampler Patrick Caddell, purportedly showing Gallen just 1 point behind Thomson, and used the poll to help raise funds for commercials on Boston television stations.

Those stations have a big audience in populous southern New Hampshire, but they are so expensive for most state campaign budgets that Gallen is believed to be the first gubernatorial candidate to "buy Boston."

Thomson professes to be unconstered by a poll made public last week which showed him leading Gallen, 43 to 30 percent. But Thomson is pursuing 16-hour-a-day campaign schedules which belie his age of 66.

Ironically, Thomson is being helped by the presence in the race of another old foe, ex Gov. Wesley Powell. Powell received 38 percent of the Republican primary vote against Thomson, without bothering to campaign, and then filed as an independent in the general election.

He is credited with getting 8 or 10 percent of the vote - most of it believed to be coming from anti-Thomson Republicans. By splitting the opposition, Powell may have the effect of helping Thomson with his fourth term.