No single theme dominates the elections this year. But they are informed by a theme central to American life - something that has been happening all over for over a decade.

What has been happening all over is the coming apart of national consensus. The 1978 election is chiefly interesting as a benchmark, a measure of whether consent continues to crumble.

The term "crumbling of consent," of course, is an impressionistic generalization. It is easier to feel than to define. It finds its most vivid examples not in politics but in the everyday life of the home, the office and the workplace. It is impossible to date with precision.

Sometime in the mid-'60s, however, what was a relatively well-established order came up against a spirit of (to use a term of the times) confrontation. Wives became more assertive in challenging husbands, children in defying parents, students in sassing teachers and workers in resisting orders from the box.

The political consequences of that change have been universal. Ins at every level have been increasingly threatened by outs. Legislative leaders have had to concede to the rank and file. Chief executives have been forced to yield to legislatures.

Twice the protest achieved the force of a national earthquake. Vietnam dominated the congressional elections of 1966 and the presidential election of 1968. Watergate unseated president Nixon and a big bunch of congressmen in 1974.

The fractions spirit continued through 1976. President Ford had to use all his resources to head off the challenge of Ronald Reagan for the Republican nomination Jimmy Carter, an outsider running as an outsider, overcame the Democratic establishment to win the nomination and the election.

As an outsider in the White House, Carter has been in poor position to curb the spirit of dissidence. For example, he has to veto the public-works bill in the name of fiscal discipline. But the political impact of the veto will be to hand knives and crowbars to younger congressmen eager to challenge the established leadership of Speaker Thomas P. O'Neill and Majority Leader James Wright.

Even before the primaries got under way this year, 50 sitting congressmen announced they would not seek reelection. With the retirement of that superb veteran, George Mahon of Texas, who is chairman of the Appropriations Committee, there will not be a single House committee led by a chairman with more than four years' experience at the helm.

As to the primary results, the most important vote by far was the victory of Proposition 13 in California last June. That was a message of protest, sent to the leaders of both parties by an electorate so angry about high property taxes that it was ready to smash the gear box of government.

Other shocks include the defeat of sitting governors in the Democratic primaries in Texas, Maryland and Massachusetts. In New Jersey, a venerable Republican, Clifford Case, lost the Senate primary to a relative outsider, Jeffrey Bell.

In Minnesota, a congressman backed by the powerful Democratic hierarchy lost the Senate nomination to another outsider. In Wisconsin, a congressman backed by the potent Republican establishment lost the Senate primary to yet another new face.

That, to be sure, is far from being a wholesale slaughter of incumbents. The vulnerable can be sharply pin-pointed. Those in trouble have tended to take the "enlightened" side on a series of narrow issues. They have been "soft" on tax cuts, on the death penalty, abortion and the environment.

But the election is far from over. Incumbent governors facing tough reelection fights include four Democrats (Hugh Carey of New York, Ella Grasso of Connecticut, Richard Lamm of Colorado and Robert Straub of Oregon) and two Republicans (Williams Milliken of Michigan and James Rhodes of Ohio).

If they go down, the election will show consent crumbling at an unabated pace. Even if they survive, the evidence suggests that, while the pace is slower and the circumstances more sharply defined, the crumbling of consent continues.