By morning after, the political professionals all had Jeffrey Bell's miracle victory in New Jersey's Republican primary analyzed down to a T.

It had to come sometime, the experts said yesterday. Cliffoord Case, the four-term incumbent whom Bell upended in Tuesday's Senate primary, state's conservative Republicans. With his passion for international affairs, Case had failed to maintain his political base at home. The senator was a sitting duck for a hard-working conservative challenger like Jeff Bell.

It made for elegant Wednesday-morning quarterbacking, but that explanation for Bell's victory had one problem: if the stage was so clearly set, how come none of the "experts" saw it coming?

Bell's narrow victory - he won by 1.5 percentage points amid a tiny turnout - was the major shocker to date of this political year. Almost nobody in politics thought Bell had a chance to win, not even his compatriots on the GOP's conservative flank. As a result, such conservative stalwarts as Ronald Reagan and Rep. Philip Crane (R-III.) had sidestepped invitations to back Bell in the primary.

Bell did profit from an undercurrent of discontent with Case among New Jersey's rank-and-file Republicans, but that alone cannot account for his victory. In previous primaries. Case had dispatched conservative challengers with ease.

The difference this year was that Bell had a remarkably popular issue - a tax cut to restrain government and spur the economy - and a remarkable ability to articulate its virtues.

Bell, a serious-minded philosopher of the right was the staff idea man in Reagan's 1976 presidential campaign. It was he who thought up Reagan's proposal for $90 billion reduction in federal spending. Although ridicule of the scheme hurt Reagan's campaign, Bell still thinks the plan was a winner if explained persuasively.

In this spring's primary, therefore, Bell took pains to explain his support for a new version of Reagan's plan: the proposal of Rep. Jack Kemp (R-N.Y.) and Sen. William Roth (R-Del.) to reduce federal income tax rates by 30 percent.

In a complex speech studded with economic jargon, Bell would set forth why he thought the tax cut, by stimulating the private sector, would actually increase federal revenues. It sounded more like a graduate seminar than a campaign spiel, but Bell found a curious reaction everywhere he spoke.

"People would come up to me afterward, dozens of them," Bell said, "and each one would tell me 'Look, I can understand what you're getting at, but that speech is too complicated for the average guy to follow.'

"Well, of course, nobody thinks of himself as average, so I decided that our point was really coming across."

Bell, who started his campaign more than a year ago, began with a crushing disadvantage in name recognition, but he worked harder and spent much more money than Case did to overcome it.

The challenger, who was born in 1944, the year Case was first elected to Congress, also benefited from his aura of youthful vigor, according to an Associated Press survey of voters leaving the polls yesterday.

The survey found that half the Bell voters were impressed with the tax cut plan, and a quarter mentioned Case's liberal stands or his age.

Bell faces a rugged challenge in November from Bill Bradley, the former basketabll star who breezed to victory in the Democratic senatorial primary, his first effort at electoral politics.

Bradley, who is also 34, has the benefit of broad name recognition and an overall Democratic majority among the electorate. On paper, he should beat Bell easily.

But on paper, Clifford Case should have beaten Bell, too.