Good news for Jimmy Carter is what mainly emerges from the announcement by Rep. Philip Crane of Illinois that he will seek the Republican presidential nomination in 1980. For Crane's early entry indicates a large field of Republicans will go for the nomination.

It also ensures a keen ideological battle with a lot of blood on the floor at the end. Republican hara-kiri, along with doing something about inflation, has to be the essence of any strategy for retrieving the Carter presidency.

At present Ronald Reagan, as he "reluctantly" acknowledged in a recent interview, is the Republican front-runner. He has an impressive national reputation, a huge personal following, a popular ideological position on the conservative side, and a corps of strong political strategists and organizers built around John Sears and Lyn Nofziger.

But Reagan would be 69 when he became president. Though he looks fine and knows how to pace himself, age could certainly wither his chances.

As a front-runner, moreover, Reagan is in what he calls a "frightening Position." The tendency is to sit on the lead and not take chances. Reagan was not out in front on Proposition 13 in California. And he did not support the conservative candidates who won Republican Senate nominations in New Jersey and Iowa.

That weakness has become apparent to other Republicans, and they are crowding the lists. Crane, for example, worked for Reagan in 1976. He told me the other day that he sees no philosophic difference between himself and the California governor now. But he plans to stick to the true-blue conservative position, avoiding the kind of compromises Reagan seems prepared to make.

Crane says that he is a "conservative first and a Republican second." He was urged to run by, and now commands the support of, the conservative fun-raiser and mail-order specialist Richard Viguerie. Indeed, Crane seems to be the conservative fallback position - an available spare if one becomes necessary, who works to keep Reagan from sliding toward the middle of the road.

Apart from being more firmly conservative, Crane is younger than Reagan, better looking, more articulate and, with eight years in the Congress, more experienced in national government. It is not surprising that he was encouraged to run by Reagan's arch-enemy, former president Gerald Ford, or that, as Crane says of the Reaganites, "they're not happy with me."

Crane's entry helps most other Republican candidates because they need a large field to get started. Those almost certain to seek the nomination include George Bush, the former CIA director, U.N. ambassador and Texas congressman; John Connally, the former Democratic governor of Texas and Republican secretary of the treasury; and Howard Baker of Tennessee, the Senate minority leader.

Bush's strong point is that he is acceptable to both conservatives and liberals, and has a foreground in Texas, where he now resides, and a background in New England, where he grew up. With a crowded field he could look very good in the New Hampshire primary.

Connally, besides glamour and ambition, has plenty of money and a strong base in Texas. In a long race his staying power could pay off.

Baker is the hope of the Republican liberals. He needs a large field to avoid a sure loss in any early one-on-one battle with Reagan.

Several other possibilities - Sen. Bob Dole of Kansas, Sen. Paul Laxalt of Nevada and Gov. James Thompson of Illinois - would all come on the scene if it looked like Reagan was stumbling. So the threat posed by Crane, dim as it may seem now, works to keep hopes alive.

Finally, there is Ford. His family and most of the Cabinet officials from his administration do not want him to run. But he is fit and popular, and he wants a role in heading off Reagan, whose strong race in the primaries probably cost Ford the election in 1976. As his encouragement of Crane suggests, Ford's role could be supporting all candidates except Reagan, and then tilting toward one of them - presumably either Baker or Bush - at the last moment.

What all this means is that a brisk contest is shaping up for the Republican nomination. With 31 primaries, it is apt to be long, drawn-out and eventually bitter. That is the best news Jimmy Carter has had in months.