Sen. John Glenn, the first American to orbit the Earth, today launched his campaign for the Democratic presidential nomination in his old home town, saying, "My life has prepared me for this challenge."

His home town, a Norman Rockwell-like place of 1,800 nestled among the rolling hills of central Ohio, gave him the kind of hero's send-off most candidates only dream of.

Hundreds of schoolchildren waving American flags lined the John Glenn Highway, 2,000 people applauded him at John Glenn High School and another 1,000 greeted him in front of the John Glenn Gym at Muskingum College.

Calling for a return to "the simple values we learned in this small town," Glenn, a plumber's son, echoed some of the same themes that helped President Reagan into the White House. But he added quickly that the Reagan administration's "deeds have fallen far short of its words."

"The policies of this administration aren't expanding opportunity, they're diminishing it," Glenn said. "They aren't promoting excellence, they're discouraging it. They aren't fostering compassion, they're reducing it.

"The president says America is on the mend," the second-term Ohio Democrat added. "I say it's time America was on the march."

Glenn, 61, was the sixth, and probably final, Democrat to enter the race formally. He did so running second to former vice president Walter F. Mondale in the public opinion polls. An April 10 Gallup Poll reported Mondale as the presidential choice of 32 percent of Democrats, with Glenn getting 13 percent.

Glenn is also second only to Mondale in fund-raising and is trying to overcome a late start in organizing. He has raised $1.1 million, compared with $2.4 million for Mondale, $809,000 for former governor Reubin Askew of Florida, $465,000 for Sen. Gary Hart of Colorado, whose campaign is in debt, $457,000 for Sen. Alan Cranston of California, and $247,000 for Sen. Ernest F. Hollings of South Carolina, who announced his candidacy Monday.

Glenn's strategy is aimed at convincing Democrats that he is a middle-of-the-road Democrat who would have the best chance of defeating Reagan in the general election. Glenn's advisers are emphasizing his straight-arrow character and experience as a pilot, astronaut, businessman and senator.

Today's announcement was staged carefully to play on his small-town upbringing and his hero status as a former astronaut.

"It's wonderful," said his media adviser, David Sawyer, surveying a picture-book scene as hundreds of students and townspeople gathered in bright sunshine on the mall of Muskingum College. "He's very comfortable with his small-town connections."

Glenn, whose father had hoped he would join him in the plumbing business, grew up here in the Depression, married his high school sweetheart, Annie Castor, the daughter of a local doctor, and attended Muskingum, a small, Presbyterian college, where he was second-string center on the football team. He left without a degree to become a Marine Corps fighter pilot in 1943.

After he became the first American to orbit the Earth in the space capsule Friendship 7 in 1962, a crowd of 50,000 welcomed him home to New Concord.

The crowd today was much smaller, but carried some of the same enthusiasm. Businessmen put up signs saying "Welcome Home John and Annie." Children rushed to get his autograph. Clapper's Restaurant advertised, "Glennburgers, 98 cents."

Glenn moved away from New Concord 40 years ago, and most of those who greeted him today didn't know him as a boy. Many said they don't know much about him other than his space flight.

His announcement speech touched all the bases dear to small-town America. He recalled that his mother was a schoolteacher and that his father, facing tough times in the plumbing business, had worked for the Work Projects Administration during the Depression. He recounted his boyhood days at a swimming hole on Crooked Creek and how he and his father played taps on their bugles at the community's annual Memorial Day services.

All this, he said, taught him "the values of excellence, honesty, fairness, compassion for those who have less, and confidence in facing the future."

"The issue is leadership," he said.

An opponent of the MX missile and a recent convert to support of the SALT II arms control treaty signed during the Carter administration, Glenn said, "The first duty of government is to keep our people alive, independent and free.

"Yes, I'll stand up for the military--and I'll also stand up to the military when that is what our national interest demands," added Glenn, who retired from the Marine Corps as a colonel.

He said he would work for "an immediate, mutual and verifiable freeze on nuclear weapons" and a reduction in "conventional weapons as well."

He called for passage of the Equal Rights Amendment, increasing federal aid to education, deferring the 10-percent cut in income tax rates scheduled to go into effect in July and repeal of the law that would index tax rates to adjust for inflation.

Glenn was elected to the Senate in 1974, on his third try. He was reelected in 1980 by a record margin of 1.8 million votes while Reagan was carrying Ohio by 450,000 votes.