Sen. Robert J. Dole, the biggest ground-gainer of all presidential candidates this past year, shows signs of reverting to the form that so stunted his previous national ambitions: nastiness and trimming.

Nastiness reared its head at an Iowa Republican affair when Dole threatened Vice President George Bush's state chairman with retaliation if he did not stop criticizing him. Trimming exposed itself when Dole backed away from President Reagan's Central American policy in an interview.

Two small incidents, even though buttressed by other shreds of evidence, will not arrest the Senate Republican leader's high flight. But they suggest difficulty sticking with advice he has rigorously followed of late: be nice and court the party's dominant conservative wing.

The Iowa incident occurred at the Polk County (Des Moines) Republican picnic June 27. Dole spotted George Witgraff, who led Bush to his 1980 triumph in Iowa and heads the vice president's 1988 campaign there. Witgraff earlier issued a puerile charge, prepared in Bush's Washington headquarters, that Dole sold out Midwestern farmers to favor tobacco growers.

That singed Dole, but not so much as Witgraff's accusation June 23 that ''his campaign staffers,'' by spreading unsubstantiated personal rumors about Bush, reinforced Dole's ''unfortunate image of the past -- a mean and nasty image.'' Four days later, an unsmiling Bob Dole fixed on Bush's chairman the hard-eyed glare that aides, a few reporters and even fellow senators have come to dread. He spoke softly so that only a discomfited Witgraff, wearing a sheepish grin, could hear. Unknown to the senator, a CNN camera recorded it all.

''I'm tired of being stabbed by you every time I come out here,'' he told Witgraff. ''If you want to play that game, I'll be glad to play it with you.'' Dole softened this slightly with his next words: ''But we don't need to play the game. We got other things to do.'' Still, the threat was palpable.

What business would a presidential candidate have down in the dirt with a a mere state campaign chairman? None, replied all Dole advisers we asked, who added it was no use nagging the senator about it. ''He saw everybody else slugging,'' a Dole insider told us, ''and didn't want to be left out.''

At the same time, the July Atlantic Monthly published an article by political analyst William Schneider. He asked Dole whether he supported the Reagan administration's ''commitment to supporting anti-communist revolutions, including the overthrow of the Sandinista government of Nicaragua.'' The response: ''I wouldn't say yes or no. Generally . . . no, I don't think so.''

The answer is unfathomable, considering Dole's sterling leadership for contra aid. His aides first told us the answer was out of context. In fact, the quote was tape-recorded in a July 16, 1986, interview with Dole and passed the Atlantic fact-checkers. Dole's men later said the controlling word was ''overthrow,'' which -- after calling the White House -- they said was not Reagan policy.

The contras might be surprised that they are not fighting to drive the communists from power. The unpleasant conclusion of conservatives, who had been warming to Dole, was that he could not resist saying what he thought a liberal interviewer wanted to hear -- an ill omen for post-nomination trimming.

Campaign advisers, explaining that they had not gotten around to discussing it with their champion, suggested we try the senator himself. He told us he was echoing the administration's position against ''overthrow,'' but added: ''I go further. I would break diplomatic relations. . . . There's no question where I stand on contra aid.''

Dole strategists worry more about the GOP's hierarchical affection for Bush as Reagan's heir apparent, which could dominate Southern primaries Super Tuesday. But the best remedy for that is confidence Dole has a better chance to win the general election, a prospect undercut by signs the Bad Old Dole is back.