"I've always thought that Massachusetts voters were the smartest voters in the country." former Senator George S. McGovern jokes in nearly every campaign speech he makes here.

Massachusetts, say the workers in the South Dakotan's campaign, is McGovern country. The Bay State was the only state in the Union to cast its electoral votes for McGovern in the 1972 presidential election. Although the District of Columbia also threw its support to the Democrat, Richard M. Nixon won that election in one of the biggest landslides in American political history.

But Nixon's demise two years later rubbed some of the tarnish from McGovern's defeat, and McGovern's one-man-band campaign--he has little money and is the only candidate who refuses Secret Service protection--has buffed the sentimental glare to a shine.

"It's been wonderful to see what has happened to him," says Michael Goldman, a Boston political consultant. "When he first jumped into the race, everyone ridiculed him as a has-been, but now you hear talk about McGovern being an important part of any Democratic administration that's elected, possibly as a Cabinet member."

McGovern, who is named with the Rev. Jesse L. Jackson as the most liberal in the field of candidates, has made Massachusetts his home since the New Hampshire primary, in contrast to the other hopefuls, who are staging their major battles in the South.

McGovern's commitment to Massacusttts is reflected by the more than $200,000 he will spend on the campaign here. He spent $58,000 in Iowa and $35,000 in New Hampshire.

Brian Best, McGovern's political affairs officer, says that with only six paid staff members, most of the campaign money is going into television and radio advertisements McGovern is relying on about 200 volunteers to canvass and man phone banks.

"This is it for me," McGovern said in an interview last week. "I stand or fall here. If I don't finish first or second. I'm dropping out of the race."

If you believe the polls, McGovern will fall here Friday's Boston Globe poll put McGovern fourth with 6 percent, far behind Sen. Gary W. Hart (D-Colo.) with 46 percent and Walter F. Mondale with 27 percent.

But, says McGovern. "I'm not paying attention to the polls. I'm sure they mean something, but I decided to run because I thought some issues needed to be brought into the debate, not because I absolutely have to be president."

One member of the staff of the Massachusetts Teachers Association, which is lending its manpower to Walter Mondale, says he is "going to vote for McGovern because he's the only one I can listen to and hear real liberalism. George has been right on the issues for so long, and he just got ruined in '72. I think we owe it to him."

Despite the likelihood of McGovern's demise today, the candidate has been throwing himself into Massachusetts and winning publicity by stridently calling President Ronald W. Reagan "the most dangerous man ever to occupy the White House."

"I feel very content," McGovern said last week. "I feel so much better than in '72 when Vietnam was tearing us all apart. I'll accept the result win or lose."