A year ago, California Republicans were confident they could defeat Sen. Alan Cranston in his bid to win a third term. What they hadn't counted on was their capacity to again nominate as Cranston's opponent the Republican candidate least likely to win.

Of Cranston, the pundits all said the same thing: that, at 66, he would be too old to campaign effectively, that he was boring, and that his inflation-racked constituents would not vote again for a big-spending liberal.

To defeat this "old, dull, big-spending liberal," the Republicans nominated Paul Gann, best-known as the coauthor with Howard Jarvis of Proposition 13, the tax-cut referendum that was approved in 1978.

Gann is a year older than Cranston, and recently inspired the Sacramento Bee to begin a story with this one-word sentence: "Boring." What's more, he has twice declared bankruptcy.

As a result, the most recent California Poll gives Cranston a 25-point lead, and the Republicans again are wondering how they let a golden opportunity slip away.

But it has always been thus for Alan Cranston.

He first won election to the Senate in 1968. His Republican opponent that year was Max Rafferty, who had first come to public attention with a speech on patriotism in which he denounced American students as "traitors and . . . slobs" whose favorite sport was ravaging little girls and stomping polio victims to death. In 1974 the senator was challenged by H. L. Richardson, an arch-conservative state senator who is remembered -- when he is remembered at all -- as the man Cranston beat by 1.5 million votes.

And now, in 1980, one might say that the third time's the charm for Cranston.

In the general election, Gann has addressed himself -- often with grammar that did violence to the English language -- to a wide range of issues, with a marked lack of success.

In August he announced his support of official relations with Taiwan. In September he angered Jewish voters with a "white paper" on the Middle East that called upon Israel to surrender the West Bank.

Gann accused Cranston of land fraud and influence peddling when it came to light that Cranston had lobbied Gov. Edmund G. (Jerry) Brown Jr. for a freeway bypass in Roseville, where a firm in which Cranston has a two-thirds of 1 percent interest owns property.

Cranston brushed off the charge by claiming he wasn't aware of the location of the property, and then totally defused the issue by announcing that he would sell his interest in the company and donate any gain to environmental causes.