If the White House was the store and the Carters were minding it, their guests, the convention delegates, were like shoppers on a spree last night -- buying everything in sight.

"Thrilling," said Diane Campbell of Sacramento, Calif., standing out on the North Portico gazing across Pennsylvania Avenue to Lafayette Park while a Marine combo played lively tunes inside. Several hundred others milled around the state rooms. They were the second batch of Carter delegates to be courted by Jimmy Carter in less than a week, and a third and final group is destined to get the presidential treatment on Aug. 1.

After that, they'll come to the convention," said a Carter aide, adding somewhat tentatively, "I certainly hope they all come."

"If anybody is wavering, you sure couldn't tell from this crowd," said David C. Garcia of the University of Washington in Seattle. "Everybody's been very very positive ever since I got there."

Positive might have been something of an understatement from the sound of the cheers, whistles and applause in the East Room a short while earlier when Carter, accompanied by Mrs. Carter, stepped onto the platform to address the throng.

"I want to ask you one question," the president began. "How many of you are convinced we're going to whip the Republicans?"

Their answer must have sounded like music to the ears of Cabinet officers and White House aides scattered around the room from the looks of the large grins that spread across the faces of Transportation Secretary Neil Goldschmidt, Education Secretary Shirley Hufstedler and congressional liaison Frank Moore.

"I think," continued Carter, outgrinning them all, "that you've got the political situation sized up very accurately."

Not that the situation was all that great politically for Carter yesterday, what with Attorney General Benjamin Civiletti's revelation that he met with the president on June 17 and briefly discussed the failure by his brother, Billy, to register under the Foreign Agents Registration Act.

Evansville, Ill., Major Michael Vandeveer said he hailed from solid Carter country, and while the president is "doing fine" there, it remains to be seen, of course, what will happen "with the investigation under the way. But I don't think it'll have the same effect as with Watergate, the two are not comparable. Plus, you have Billy Carter. And you have to take that into account."

Taking Billy Carter and his Libyan connections into account didn't seem to faze Don Sonntag of Atlantic, Iowa. "Most people have a certain sympathy for the president," Sonntag said.

Some delegates, like Diane Campbell and fellow Californian Al Murray, pooh-poohed talk about erosion in support for Carter. "No second thoughts," said Murray, an analyst for Californian's state welfare department.

David C. Garcia of Seattle noted that the reason he was there in the first place was that "I was elected by the people back in Washington as a Carter delegate."

He said some of the delegates sounded him and others out yesterday on whether they would support native sons -- "you know, Sen. [Henry] Jackson's name has been memtioned" -- but it was nothing organized, "nothing official."

Phil Davitt of St. Charles, Iowa, minority leader in the Iowa House of Representatives, said rising corn prices and increased hog sales have given Carter's candidacy there a sizable boost in recent days.

"The mailman told me just the other day that there's been a tremendous change in attitudes the past month and that people he talks to are coming back to Carter. A month ago they were really angry."

Davitt got the VIP treatment Thursday when he arrived in town from Iowa. He told of telephoning the presidential assistant Stu Eizenstat's office from Iowa asking help in setting up meetings with top officials in the departments of agriculture, transportation and the treasury in order to discuss what the federal government can do to help two bankrupt Iowa railroads.

"I think we may have some railroad aid coming -- certainly, I got the impression they were considering it," said Davitt, who thought it unlikely that "they'd let me fly to Washington, talk to three departments and then do nothing."

Jimmy Carter was also looking for a little help, and he told the delegates:

"I trust I will be making an acceptance speech to you in New York. In preparation for that moment, please remember we have strong Democratic Americans who support Senator Kennedy for president. . . Remember that after the campaign, if things go the way we want them and expect them to go, they will be political allies fighting the same battle."