NEW HAMPSHIRE'S Gov. John H. Sununu, the lively new head of the Republican Governors Association, uses a tease line from an old hair product commerial to fortell the politics of 1986: "Which twin has the Toni?"

By that he means, voters beware: Look-alike, sound-alike candidates will be falling all over themselves next year to lay claim to the same centrist rhetoric and pragmatic policies.

Sununu was talking about governor races, but his witticism seems likely to apply across the board, according to office-holders, pollsters and campaign consultants.

From both ends of the political spectrum, there has been a collapse to the ideological center in the past year -- or, perhaps more accurately, a drift away from ideology altogether. Democrats are sounding more like Republicans. Republicans are sounding more like Democrats. The overarching political debate of the past decade between the two parties -- a debate about the role of government itself -- appears to have run its course.

The Republicans won. But victory itself always sets the stage for a new, different debate.

The terms of the post-Reagan argument have not yet been fully defined, and in this time of transition, the prevailing political instinct appears to be toward the safe, neutral harbor of pragmatism. "We are in sort of a let's-make-it-work period, and if it's working, let's keep stoking the fires," said Democratic pollster Peter Hart.

Republican pollster Robert Teeter told a recent Republican Governors Association meeting that his data shows voters are less plugged into public affairs than they were a few years ago. In a time of peace and prosperity, government itself -- for better or worse -- has lost some of its salience.

He added that the anti-tax mood is not as strong as it had been in the late 1970s and early 1980s; that voters are now more willing to pay for government as long as they are convinced their money is being used efficiently.

"We are moving into an era when people do understand there is a role for government," GOP consultant Lee Atwater told the same group.

Both men, suprisingly, were preaching to the choir. Govermental activism -- the very phrase was an epithet at GOP gatherings a few years ago -- is the order of the day in Republican governors' mansions now. With the "smothering excess of government from Washington" now curbed, "there has been an explosion of energy and creativity and problem-solving" by Republican governors, said a slick booklet, "Reagan Stage II, the State and Local Revolution," prepared for the conference.

The report spotlighted a state "workfare" program in California, a toxic-free environment plan in Delaware, small business incubators in Illinois, overhauls of the public education system in Missouri and Tennessee, public, private and higher education research partnerships in Pennsylvania and Rhode Island, and a minority opportunity program in New Jersey, among others.

A similiar inventory of initiatives launched in recent years by successful Democratic governors would be virtually identical.

So what's the difference between the two parties at the state level? Sununu, with his fondess for television advertising metaphors, argues that Republicans are the "real thing" -- the original advocates of government close to the people and the true champions of private-public partnerships.

But these are rather fine distinctions, unlikely to sway voters. Just as the Republicans are now talking about more government, the Democrats are now talking about less, and the two parties seem to have stumbled over each other in the general rush to the center.

If the rhetoric of the two parties doesn't offer a voter much to choose from these days, the chief political and legislative battles of 1985 didn't offer much guidance either.

Consider the two gubernatorial contests of 1985: In New Jersey a progressive Republican, Gov. Thomas Kean, swept to a second term with an astonishing 60 percent of the black vote; in Virginia, a Democratic rainbow ticket of a conservative white, a black and a woman won handily -- talking about economic growth and tough measures against crime.

Meantime, in Congress, compare 1981, the first year of Reagan's first term, with 1985, the first year of his second. Four years ago, a readily identifiable coalition of Republicans and conservative Democrats wrote into law the Reagan economic program, setting the stage for the sharp ideological debates of 1982 and 1984.

This year? The most important piece of domestic legislation in 1985, the Gramm-Rudman-Hollings deficit reduction measure, was an initiative of the congressional GOP supported by half the Democrats in Congress, opposed by the other half (with no easy ideological divide between the camps) and signed into law by a tag-along president.

The other centerpiece domestic bill, tax reform, struggled its way through the House only after Reagan lined up votes from balky members of his own party with a promise that he would veto the bill unless the Senate next year removes key Democratic alterations to the president's original tax proposal.

Got that? The congressional year thus ended last week with Democrats convinced they will be able to score points on the tax issue by casting themselves as the champions of the little guy on taxes, and with Republicans believing they had seized the high ground on the deficit problem.

But both these seem to be small positioning gains, lacking in clarity and punch to most voters, and likely as not to vanish with tomorrow's headlines.

So for now, the question recurs: Which twin has the Toni?