He got his start, he says, as "sort of the mascot" of the Reagan administration.

He is 25, but looks all of 17. Stands 5 feet 7, weighs 127 pounds, and wears Coke-bottle-thick glasses that cover most of a small, peach-fuzzy face.

He is running for Congress in a northeast Pennsylvania district that ranks seventh in the nation in percentage of senior citizens. He's knocking on 300 doors a week, trying to establish a "grandparent-grandson" chemistry with the voters.

After the 1980 election, he briefly shared a Washington, D.C., apartment with Reagan campaign officials Edward Meese III (now attorney general), William J. Casey (now CIA director) and Drew Lewis (later transportation secretary). He was "the one who went out for pizza." After that, he did a two-year stint as executive director of Citizens for America, a conservative, grass-roots lobbying organization launched by drugstore magnate Lewis Lehrman with the encouragement of President Reagan.

His administration connections, starting with his old friend the president, are an embarrassment of riches. He is not bashful about tapping them.

At a $1,000-a-couple reception and "insiders issues briefing" his campaign threw yesterday at the Mayflower Hotel, four Cabinet members, a couple of dozen senators and congressmen and a platoon of White House staff members had to fight for top billing.

"I have never seen such a list of people come together for a sitting member of Congress, much less for a challenger," said Richard V. Allen, former national security affairs adviser, who hosted the event.

Nobody in politics knows quite what to make of Marc L. Holtzman, 25, the gofer-turned-operative-turned-candidate, the wonderboy of the Reagan Revolution, the state-of-the-art candidate of the 1980s.

He has raised $360,000 toward his 1986 campaign: No one in the Republican Party establishment can remember any congressional challenger collecting so much so early. He has done it by plugging into the Reagan fund-raising network, by hunting down Texas oilmen for three weeks this summer, by touching up business associates of his father, Seymour Holtzman, who owns Jewelcor, a national catalog sales company based in Wilkes-Barre, Pa.

He says he was shy about asking for money for himself until Richard DeVoss, CFA's finance chairman, cured him of the affliction. "He told me you have to know in your mind that you are doing the contributors more of a favor by running than they can ever do for you by giving," Holtzman explained.

That quality of naked aggressiveness, which serves as a foil to his youthful earnestness, has been tickling the fancy of the old guard of the Reagan crowd since Holtzman started as a volunteer in the 1976 presidential campaign.

From 1976 to 1980, candidate Reagan used to charter the Jewelcor airplane for trips into Pennsylvania, and young Holtzman would tag along. "When you're cooped up together in a twin-engine plane, you really get to know somebody," he says of his friendship with the president.

The story most often told about the two is of the time Reagan, then the GOP nominee, had decreed that Holtzman, at age 20, direct his 1980 presidential campaign in Pennsylvania. Reagan counseled the young man that he would be giving orders to people two and three times his elder, and that he needed to handle the age issue with sensitivity. "Guess we have the same problem," Holtzman is said to have shot back. (He tells a slightly different version.)

Now Holtzman is facing a more daunting challenge. Can his money and his Washington credentials overcome the fact that he is a Jew in a 67 percent Roman Catholic district, a Republican in a 57 percent Democratic district, a "daddy's little rich boy" running in a corroded corner of the Rust Belt?

Convential wisdom sets long odds. "He better raise all that money; he's going to need every penny," said Ed Mitchell, consultant to Rep. Paul E. Kanjorski (D-Pa.), the first-term incumbent who must defend his seat against Holtzman.

The Washington-based business/political community so far has shied away from joining Holtzman's contributors parade, staking out its usual give-the-nod-to-the-incumbent ground. But Holtzman knows a few of the tricks. He has listed all of Kanjorski's PAC donors on a computer. Whenever the Democrat casts a vote against any of their interests, Holtzman drops a note to the appropriate PAC manager.

As to the religion question -- if you're inventive (and connected), there are ways of handling it. This summer Holtzman paid a visit to his old pal William A. Wilson, ambassador to the Holy See. Wilson was able to oblige Holtzman his one request: a meeting with the pope. Photos ran in several newspapers in the district.

Pennsylvania's 11th District has been the nation's most volatile. It has had five congressmen in the past six years. Observers attribute the turnover to the the legacy of former Democratic representative Daniel J. Flood, who created a standard of constituent service that his successors have been unable to match.

Holtzman campaigns as a hard-liner on defense -- he's espcially fervent about the Strategic Defense Initiative and aid to the counterrevolutionaries in Central America -- but he also is running hard on the issue of service.

As a candidate, he has already led a trade delegation to Taiwan and a delegation of footwear manufacturers to Washington to lobby for import restrictions on shoes.

The shoe gambit didn't work out as hoped. Holtzman says he was "taken aback" by Reagan's decision not to impose quotas, and knows his high profile on the issue may undercut his argument that he would be a can-do man on the Hill.

But he plugs on, 14 months out, with an intensity and single-mindedness that takes away the breath of his seniors.

"I have never seen anybody like Marc," said Joe Gaylord, director of the National Republican Congressional Committee. "He is the textbook candidate. You suggest that he do this, and he's already done it. You suggest he go there, he's already been."