BOSTON — The midday sun illuminates a crowd of campaign staffers and volunteers carrying blue and white “Deval Patrick” signs. In their midst stands the candidate, Mr. Patrick himself, in a light blue shirt and crisp tie. He has spent the last hour in Downtown Crossing, the old retail district, shaking hands and greeting people on their lunch breaks. He concludes his words to supporters with a reminder that is unnecessary: Tuesday is Election Day.

“Bring it home tomorrow,” he says, to the crowd’s delight.

Mr. Patrick, who served as President Clinton’s assistant attorney general for civil rights, is one of three gubernatorial candidates competing for the Democratic Party nomination today. The others are the state’s attorney general, Thomas Reilly, and a venture capitalist making his third major run for elective office, Christopher Gabrieli.

The winner will face off against the Republican lieutenant governor, Kerry Healey, an unconventional independent, Christy Mihos, and a Green candidate, Grace Ross.

The campaign has been close and hard fought. In the last seven days, polls have shown a late-breaking surge of support for Mr. Patrick, but because the independent voters who make up more than half the state’s electorate are allowed to vote, today’s result is still uncertain.

While policy differences exist among the three Democrats, a central question hangs over the entire primary: Which candidate presents the best chance for the Democrats to put one of their own into the governor’s office on Beacon Hill for the first time in nearly 20 years?

How Republicans have maintained control of the executive branch of the commonwealth’s government for almost two decades is the great paradox of Massachusetts politics. To outsiders, it is all but inconceivable that Republicans, who command but 13% of registered voters in the state, dominate the top office in the state. The Bay State, after all, is a place defined in the national mind by its Senate delegation, John Kerry and Edward Kennedy; high-profile congressmen, Barney Frank and Martin Meehan; and far-reaching social change, such as the legalization of marriage rights for gays and lesbians.

The reality is that for decades state elections have run on an entirely parallel track from national contests.

The key distinctions are fiscal. In 1980, Massachusetts became only the second state to vote for a measure limiting property tax on the local level, Proposition 2, modeled after California’s Proposition 13. Ever since the advent of Route 128 as “America’s Technology Highway,” when Raytheon, Digital Equipment Corporation, and other high-tech companies grew up in the outskirts of Boston, the successful candidate has been the one who can best win over the suburban professionals who live between the two beltway highways, Route 128 (now 95) and Route 495.

It is these voters — many of them among the 51% of unenrolled or independent voters — who have elected, in succession, Governors Dukakis, Weld, Cellucci, and Romney. They are not, however, the voters who will win the contest today. The top election official, Secretary of State William Galvin, is predicting a turnout of a mere 600,000 voters, roughly 180,000 voters fewer than those who came out during the last election cycle.

The conventional wisdom suggests this should help Mr. Patrick, the first state candidate to win over the so-called Netroots, the community of bloggers and progressive activists whose energy is now driving national Democratic politics.

Another scenario holds that the low turnout will help Mr. Reilly, who has the support of the major unions and big city mayors, including Mayor Thomas Menino of Boston. If the attorney general is within 3% to 5% of Mr. Patrick, his get out the vote operation could make the difference.

And, just days ago, Ms. Healey, the Republican, began running TV ads attacking the suburban-friendly Mr. Gabrieli, a classical “New Democrat” of the Clinton model, in an apparent attempt to discourage independents from going to the polling stations on an election day — a sure sign that she sees him as her biggest threat.

Early in his campaign, Mr. Patrick met with Mr. Dukakis for advice on his statewide run. The former governor, among others, told Mr. Patrick to build his campaign from the grass roots, and the candidate apparently has updated that model for the Internet age. (One of Mr. Dukakis’s legacies is his meticulous focus on the grass roots, which has rendered his protégés and former staffers among the most sought-after political operatives each presidential election cycle. Mr. Kerry, for example, would not have won Iowa and hence the Democratic nomination in 2004 without the support of this school.)

Mr. Patrick’s candidacy does resemble Mr. Dukakis’s in an important way. When Mr. Dukakis, a graduate of Swarthmore College and Harvard Law School and a resident of Brookline, won the Democratic nomination for governor in 1974, he defeated the state’s attorney general, Robert Quinn. Mr. Dukakis played the advocate of good government against his opponent, an urban Democrat and former House speaker. In a statewide race, Mr. Patrick would likely campaign on his status as a stranger to the state’s clubby political culture.

“I’m an outsider,” Mr. Patrick, taking a break from shaking hands, said. “A whole lot of us believe that the ‘same-old, same-old’ doesn’t work anymore.”

In a poignant footnote to today’s election, just as Mr. Patrick made the rounds in Downtown Crossing, word came in that a key figure in state politics, Ed King, had died. King was the last prominent example of another Massachusetts anomaly, the conservative Democrat. His defeat of Mr. Dukakis in 1978 — again on the backs of those suburban independents — prefigured the statewide tax revolt in 1980.

A win for Mr. Patrick today will take the party further away from the legacy of King and closer to the national Democratic mainstream. But whether the winner is Mr. Patrick or one of the other candidates, the next governor of Massachusetts will be the candidate who can win in the suburbs.