Ruth Clark stood in a puddle of melted snow in a vacant lot along with about a thousand others one day last week and waited to give a hero's welcome to her candidate for president.

John B. Anderson, suddenly transformed into the man of the moment and the momentum, used to drive her kids to nursery school when the two families were neighbors 20 years ago. "He was a terrible driver," she confided. "I'm glad now he'll get a limousine."

Next to her stood Nels Akerlund, a businessman who remembered when Anderson was his Sunday school teacher 25 years ago. Others said they were friends of his 95-year-old Swedish immigrant father, Albin, who sat on the platform.

It was the kind of down-home loving turnout that a favorite son expects from his friends and neighbors. But, as the candidate himself might put it, the scene told only a half truth about voter sentiment at home.

Unlike Sen. Edward M. Kennedy in Massachusetts, or President Carter in Georgia, Anderson has to share his backyard.

In a gaudy display of political abundance, the humble farm-and-factory-land of the 16th Congressional District alone produced not one but two native sons running in Tuesday's Illinois primary -- Anderson and Ronald Reagan, who was raised in Dixon, 30 miles to the south.

Folks have neighborly memories of Reagan, too. Moreover, to them he's a "real Republican," and he's been a presidential prospect for much longer than Anderson.

Long before Anderson's unexpected surge, a poll conducted last October for the Illinois state Republican committee showed Reagan with 48 percent to Anderson's 12.

Philip M. Crane, born in Chicago and now representing the suburban 12th District in Congress, makes a third -- though fading -- favorite son presidential candidate from Illinois.

The expected home-court edge in Illinois is further confused by conflicting loyalties and old tensions, by Anderson's controversial departures from traditional Republican thinking, and by the "blind primary" ballot that does not identify which Republican candidate a given delegate is pledged to support.

The state's leading newspapers have endorsed Anderson, and many of his friends and neighbors -- Democrats as well as Republicans -- are enthusiastic supporters.

But others forecast a neck-and-neck finish between him and Reagan even here in Anderson's home district.

"There's a certain amount of hometown pride in Anderson, but it's starting to wear a little thin" after last week's Republican debate, said W. Timothy Simms, Reagan's coordinator here, a funeral director and state legislator.

Anderson's refusal to pledge his support for the Republican nominee next fall, and a widely circulated fund-raising letter Anderson signed in support of liberal Democratic members of Congress, have troubled many Republican loyalists.

"The Rockford businessmen are absolutely appalled at the possibility Anderson might be president, most of them," said Dave Martenson, Anderson's old law partner, manager of his first campaign, and now secretary of the Illinois GOP state committee. He is no longer as close to Anderson as he once was.

Some of the antagonism toward Anderson is left over from his 1978 congressional race against an ultra-conservative minister, when some Republicans charged that he had forgotten about the folks back home. Anderson won, according to some, only because his opponent was so "unacceptable."

The former opponent, Don Lyon, said last week "When I heard John B. accuse [George] Bush of telling half truths and innuendoes, I said, 'I've heard that line before.' That's the way he always tried to brush my criticism aside . . . But those 'half truths' he accused me of talking last time are whole truths now."

Reagan is generally conceded to be much stronger than Anderson to the south of Woodford County, site of his alma mater.

"A helluva lot of people" remember Reagan, said Sam Harrod, a former classmate of Reagan and now his Woodford County chairman. "They've been waiting for him to be president for years."' Indeed, in true Doonesbury style, Anderson has only one high school girl, Laura Lamb, phoning for him in Woodford, according to his coordinator in the 15th District.

Bush, the outsider in all this, has made only minimal efforts to combat the favorite-son syndrome in Reagan and Anderson strongholds.

Despite the strong antagonism Anderson stirs up, a lot of his Republican neighbors as well as Democrats and independents reportedly share the sentiments of Gary Schowalter, an engineer. He paused over a beer at a Rockford bowling alley to talk about it.

"I'm so disgusted with politicians it's difficult to get exicted about any of them. But I think John B.'s comin' on."

Schowalter said that the fact that Anderson is a "bad Republican" is "an asset. I don't think the voting public is party oriented. Anderson is the only one saying anything."

Art Mumm, a Rockford businessman, said he voted for Reagan when he was in California and still admires him. "But John B. is smarter than all the rest of us. I just think he'd make a better president."

At a Republican dinner here a few nights ago, Winnebago County party chairman Jim Kelley looked around and said, "this is a conservative county but . . . there are a lot of new faces and new money at this dinner because of John. You can't underestimate hometown pride."

Later that night, when Anderson had the audience nodding off with an earnest but vague and cliche-ridden speech about how he would run his presidency, an oldtimer out in the crowd periodically startled them awake by leaping to his feet and yelling, "Let's give John B. a hand on that!"

And the home folks obliged, every time.