When Andrew P. Miller ran his Democratic primary race against Henry E. Howell this spring, he made so many mistakes that some Virginia politicians cite the race as a classic of how to run against Howell, the flamboyant populist.

Two of Miller's mistakes, many Howell and Miller strategists now agree, was to underestimate the impact a Georgian named "Carter" could have on the race, and the other was to duck the chance to debate Howell on television.

Last week, for reasons quite similar to Miller's, Republican Lt. Gov. John N. Dalton did the same. Not only did he minimize the impact of President Carter's planned swing into the state, but he encouraged Virginians to go see the President. ("Dalton believes it's always an honor to visit Virginia," a Dalton spokesman said.)

Earlier Dalton abruptly cancelled plans for future joint appearances with Howell, citing "the type of personal attacks" Howell has made on him. No doubt Dalton was stung by some of Howell's remarks which have likened his direct - mail advertising to Nazi propaganda and have described some of his letters as "meaner than a junkyard dog."

Still, in view of what happened to Miller's $1 million campaign and what appears to have been an extremely well - executed seven - four hour foray by the President on Saturday. Dalton might be rethinking his approach to the fall campaign.

Surprised though his staff was by the President's direct charges at Dalton, they were hoping this week to turn Carter's remarks into a major issue. "Even Jimmy Carter can't endorse Henry Howell with a straight face," claimed William ROyall, Dalton's manger.

What delighted Royall was that Carter's ringing endorsement of "my personal friend" came with some qualifications, principally the statement that "... one of the main concerns about Henry Howell is that he sometimes is indiscreet.

Carter urged his Roanoke audience to forgive Howell's past indiscretions and Howell's tendacy to "create controversy." As the President saw it, in a politician "that is not anything to be ashamed of."

But Virginia Republicans are not in a forgetting or a forgiving mood, especially when it comes to Howell. "We intend to make certain that the people of Virginia know about those indiscretions," Royall promised this week.

Miller also thought Howell "indiscreet" but his campaign never seemed able to project that image to the state's Democratic voters."We intend to make sure that the people of Virginia know about those indiscretions," Royall replied when asked if Dalton planned a "negative" campaign against Howell.

Although such campaigns are difficult and risky to mount, Howell's staff has described them as standard operating procedure in Republican campaigns and have said they expect a major Republican broadside just before the Nov. 8 election.

Brief though Carter's visit was Saturday, it seemed certain to have raised the WhiteHouse's stakes in the race and make certain at least one more visit by Carter into the state. If nothing more, some Howell strategists would like to get the President into the Northern Virginia suburbs for a high school gymnasium rally in an area that both Republicans and Democrats concede is crucial in the election.

Whether Carter can have the impact there that he appeared to have over the weekend in Roanoke Northfolk and Williamsburg is debatable. Virginia is the only Southern state that the President failed to win last year, and the county's well - orgnaized Republicans in populous Fairfax County are as much the cause of his Virginia defeat as anything in the state.

Turnout Carter received in Roanoke, about 30 miles from Dalton's Radfod home was big(10,300 in the city's civic center) and enthusiastic, surprisingly more so than the 4,000 who turned out for the President in Howell's hometown of Norfolk. The crowd there was well below the 10,000 that some Howell supporters had hoped to draw.

Republicans have not heard the last of the three speeches Carter made for Howell. Trailing Howell and the President throughout the day was a camera crew from the same Arlanta advertising that handled Carter's 1976 campaign.

If the Bert Lance affair has damped the President's popularity as Dalton has predicted that was not apparent at any of Carter's Virginia stops. As Howell put it in Roanoke prior to the President's arrival: "The people in the rural areas in Virginiareally thought they (Lance critics) were really trying to go at Jimmy Carter and that they were overly rough on Lance...There is a lot of Calhoun, Ga, (Lance's hometown) in Virginia."

Although never mentioning Dalton by name, Carter set the tone for his involvement in the Virginia race by berating candidates who won't make full financial disclosures and won't debate, as Dalton won't.

Recalling his Williamsburg debate with President Ford, Carter said in Roanoke his meetings with Ford may have been crucial to his campaign."Nobody knows" if the debates were, he said. "But I do know one thing: I didn't have anything to hide. I was willing to debate." Apllause interrupted Carter.

"My understanding si that Henry Howell is not hiding, he is willing to debate," the President added, to another burst of applause.

Those are words the Howell campaign can be expected to stress repeatedly in Northern Virginia, where both candidates tend to suffer from greater name recognition problems than anywhere else in the state

If, as one of Richmond's conservative newspaper suggested last week, "The issue is Howell" in the gubernatorial campaign, then John Dalton faces the task of convincing many of the state's undecided voters that not only is Henry Howell a dangerous man, but that the President also has badly judged the character of his "long - time friend."

Neither is an easy job, as Andrew Miller could have toldJohn Dalton on June 15.