Events, as the ups and downs of Jimmy Carter demonstrate, determine politics with a vengeance these days. So, genuine unknowns, true X factors, obscure the outcome of the presidential primaries and the general election.

What does emerge clearly, however, is the spirit of each of the two parties.

While the Democrats make faces at each other, the Republicans turn a friendly countenance to the world, and that difference is apt to say a lot about eventual winners and losers.

Democratic nastiness begins in the White House Jimmy Carter personally mocked Sen. Edward Kennedy for seeking his mother's assent to his candidacy, A cruel jest that, at the expense of a woman who has already lost three sons dedicated to the service of this country.

Bob Strauss, the Carter campaign manager, could hardly wait after Kennedy's foolish comment about the shah to intimate that the senator was playing fast and loose with the lives of the hostages G. William Miller, the secretary of the Treasury, raises money on Wall Street by hinting that Kennedy would make the country go bust. And in all quarters of the Carter camp, tasteless jokes about Chappaquiddick are the order of the day.

Kennedy partisans stoop just as low to conquer. A steady whine of unsubstantiated complaints has Jimmy Carter hiding behind the Iranian crisis while working round the clock on the phones to line up local officials from alderman up. Evidence of national difficulties -- from rising prices to mounting unemployment -- finds a ready welcome. Dirty cracks about the president's religiosity abound.

The third man, Jerry Brown, lumps Kennedy and Carter together as lackeys of the oil companies. So far from providing a buffer, he only rubs up the bitterness. His great hope, indeed, is that between the two leading Democrats there will occur political fratricide.

One Republican sounds as mean as the Democrats. John Connally, the last of the true-blue New Dealers, speaks in the grimmest accents of the need to project military and economic power against a hostile world. But so far, a major feature of the Republican race is the failure of Connally to get moving.

Especially when Connally is compared with two nice guys in the running. Ronald Reagan, of course, is one of these. He comes on easy as an old shoe -- smiling and slightly diffident, the ordinary citizen reducing the mysteries of high policy and great office to one-liners comprehensible to just plain folks. Everything he knows seems to be on the tip of his tongue. But that doesn't get in the way of a brilliant strategy. Not for three decades has the country seen an early front-runner of the out-party who was so likely to go the distance.

George Bush, an equally nice guy, provides the other success story on the Republican side. Bush comes from a long line of Connecticut patricians whom Republican conservatives used to eat for breakfast not very long ago.

To defend himself against charges of being a moderate, Bush has had to go far out in opposing such matters as the Panama Canal treaty and the arms control accord with Russia. But he is so genuinely affable, a fellow so winning in his ways, that in his mouth the threat of World War Iii sounds like the center court at Wimbledon.

It may well be that the nice-guy stance pays off in the Republican Party. With Nelson Rockefeller gone, a bone is out of the party's throat. Resentment runs low. The genial mood is against John Connally in his effort to make the country take its problems seriously. It protects Ronald Reagan against punches at the glass jaw of age. It makes it thinkable that -- if Reagan stumbles -- Bush could actually win the Republican nomination.

Whether the mood of the country jibes with that of the GOP is not a bit clear. But my own sense is that the voters prefer smiles to frowns, bromides to home truths. If so, the Republicans enter 1980 in good shape to challenge whatever winner emerges from a Democratic fight that is more and more shaping up as a long war of attrition.