Mike Petro, one of those serious young men who seem to have been born wearing a gray suit, is part of a new Washington growth industry. He is a PAC-Man.

He works for Florida Gov. Bob Graham's Senate campaign out of a cramped little office on Capitol Hill.

There are 4,000 political action committees, or PACs, registered in Washington. There are big PACs, little PACs, PACs for doctors and nurses, farmers and firefighters, lawyers and bricklayers, business and labor.

Each was formed for the same purpose -- to use money to curry political favor -- and each has its own political war chest. Together, PACs contributed $105.3 million in House and Senate races during the 1984 election cycle.

Petro's job is to get as much PAC money as possible for his boss. The competition is tough. "Washington fund-raising is a campaign in itself," said Keith Abbott, finance director of the Democratic Senatorial Campaign Committee (DSCC). "I'd say most of our candidates have one, two, three or even four people working full time for them in Washington. They're dealing with 4,000 PACs and it takes a lot of work."

Republicans, of course, have their own PAC-Men and PAC-Women. Democrat Graham's opponent, Sen. Paula Hawkins (R-Fla.), has contracted with PM Co., a Washington-based firm, to raise PAC money and stage major events. Hawkins, like Graham, has a Washington steering committee to help.

The idea, said Petro, who raised money for the presidential campaigns of Sen. John Glenn (D-Ohio) and former vice president Walter F. Mondale, is "to personalize your appeal, to make people in Washington feel like full partners in a campaign taking place in Florida."

To do this, Petro, 27, sets up "meet and greets" with political money people when Graham is in town, talks with PAC leaders by telephone, issues hundreds of invitations to Graham fund-raising events, establishes a phone bank to follow up on the invitations and "papers PACs with information."

About every six weeks, a campaign "update," signed by campaign manager Jim Eaton, is mailed to selected "PAC leaders" with a glowing account of Graham's efforts.

On May 18, for example, Eaton wrote: "We continue to be encouraged by the positive direction of the Graham for Senate campaign. A poll published May 18 by Cox newspapers in Florida reaffirmed Graham's healthy lead. Fund-raising is one target." A clipping of the poll, which showed Graham leading Hawkins by 13 percentage points, was attached.

This is an integral part of PAC priming. PACs tend to follow the pack; few are venture capitalists. "Washington money, by and large, is smart money," said Abbott of the DSCC. "Most PACs are not a bit interested in supporting people who they don't think will win."

Petro doesn't bother with all PACs. Some give only to incumbents, others only to Republicans. He concentrates on those with a history of giving to challengers, especially Democrats. About 1,100 PACs fall into this category. Under federal law, each is allowed to contribute a total of $10,000 to a candidate in the primary and general elections.

Petro has helped organize four Washington fund-raising events for Graham. The most recent was at the Sheraton Plaza Hotel June 26. Six weeks earlier, Graham met over breakfast with about 40 PAC leaders, lobbyists and other supporters who had pledged to help sell tickets to the fund-raiser.

Lobbyists and PAC leaders treasure such intimate events. "I try to give smart money," said John McEvoy, who decides how the PAC formed by his law firm, Kutak, Rock & Campbell, spends its $45,000 a year. "What's nice is sitting down at a dinner or over breakfast with a candidate."

"PACs expect to be worked personally by a candidate," said Brad O'Leary, president of PM, Hawkins' PAC firm. "What they want you to say is, even if the candidate doesn't agree with them, that they can communicate. If you don't begin communicating before you're elected, you're not going to communicate after."

A modest crowd of about 75 people, including representatives of a host of labor PACs, attended Graham's June 26 fund-raiser. Graham gave a short speech, chatted with guests for about two hours and then flew to New York for another fund-raising event.

The Washington event, Petro said, had raised $170,000 in pledges. He was pleased.