DES MOINES -- Sen. Robert J. Dole's surge in crucial Iowa is remarkable because he has not fully corrected shortcomings that have plagued him as a national candidate for more than a decade.

While he has stopped being his own campaign manager, it may be temporary (''I think's he's just on vacation,'' quipped an old friend). He is so undisciplined he still will not deliver a prepared speech. His tart tongue, mostly held in check, occasionally lashes an opponent or even a voter. He remains the consummate legislator, wary of taking positions on anything (including the INF treaty) until the Senate roll is called. Most of all, he cannot resist calls for ''belt-tightening'' and ''medicine'' that smack of austerity.

Yet Dole has surged ahead of George Bush, once thought to own Iowa. Private surveys and tracking show the NBC poll's 16-point Dole lead is only a little exaggerated.

The vice president's operatives concede they are behind in a state neither can afford to lose. Thanks to help from Sen. Charles Grassley, Dole's organization has moved from a dead start last spring to even or ahead of Bush's. There is grass-roots talk of a possible third-place Bush finish here, behind Pat Robertson's shadowy army.

The explanation of Bush's managers is that this is farm country long hostile to Ronald Reagan. But our questioning of rank-and-file Republicans, some committed to Dole and many not, who heard the senator on a recent day-long swing through Iowa provides broader answers. They describe Dole as ''more electable,'' decry Bush as ''too liberal'' and talk of the Kansas senator as ''one of us.''

Dole, inside-the-Beltway darling of the lobbies, is painting a conscious self-portrait as a conservative populist of the prairies. It seems to be working, even with lapses in self-discipline.

In an impromptu television interview at a Farm Bureau pancake feed in Des Moines, Dole snapped that Bush did not know anything about agriculture -- quickly adding, ''I don't mean to be critical.'' But wasn't that being critical? ''It's just a fact. I don't do that {criticize} anymore.'' Was he taking advice to be nice? ''For a while,'' he replied, quickly adding: ''I've always been a nice fellow.''

Whenever he was asked a pointed question during the day, Dole tended to fire back hard. When a listener over a WHO radio call-in asked about his tax-raising record, he replied: ''Tell Jack hello for me. You sound like you're calling from Jack Kemp's headquarters.'' At a town meeting in LeMars, when a young Democratic farmer challenged him, the senator snapped, ''You're right about everything, aren't you?''

The biggest breakdown in discipline came at the state meeting of electric cooperatives here, where he was supposed to deliver the day's only prepared speech. He read not a word. Instead of giving his basic campaign speech, he wandered through a 35-minute monologue that became increasingly incoherent as he frantically flipped pages in his speech notebook trying in vain to pick up the text.

Worse than artistic failure was his return to the pain-and-suffering theme from which his new campaign manager, William Brock, has weaned him. He has been told to eliminate talk of ''sacrifice'' and his holding the spoon for ''bitter medicine.'' He has been told to say the government, not the people, should tighten belts.

But in his meanderings before the electric co-op executives, this came out: ''We have to tighten our belts a little. We need medicine, and somebody has to hold the spoon.'' In Sioux City, he said, ''We're going to have to sacrifice a little . . . I'm not preaching austerity, . . . {but} we've got to take some medicine.''

Still, apart from conditional support for an oil import fee, Dole backs away from his taxer's image (''I don't find many Americans who think they're undertaxed"). He also shelved his old dirge of economic trouble ahead, saying that apart from the budget deficit, ''everything else is going along fine.''

He is short on specifics, but nobody outdoes Dole's standard stump speech today. His intimate account of overcoming poverty and war wounds transfixes audiences. Despite occasional reversions to peevishness and negativism, he comes over to Iowans as a fellow plainsman who promises, ''I won't forget where I come from.'' For George Bush, that spells trouble.