PACs, parties, even candidates pump big money into '84 races.

With a stunned look on his face, John Scott hung up the phone and sat down heavily in a handy chair. In Washington for political meetings, the executive director of West Virginia State Republican Party had just discovered how much the Democratic Senate candidate in his state was spending. The Democrat's name? Rockefeller. John D. Rockefeller IV.

''Millions,'' Mr. Scott muttered to a reporter. ''Millions.''

With four months to go until election day, a Mississippi River of money is already flowing into US politics. An analysis of midyear reports just released by the Federal Election Commission (FEC) reveals that:

* John D. (Jay) Rockefeller has pumped $4.2 million of his family fortune into his race for an open Senate seat. He easily wins the award for Candidate With The Deepest Personal Pockets.

* Sen. Jesse Helms (R) of North Carolina has already surpassed the champion Senate fund raiser of 1982, Pete Wilson of California. As of June 30, Mr. Helms had raised $9.1 million to aid in his reelection bid.

* Political action committees (PACs) are spending more than ever. From January 1983 through June of this year they contributed approximately $50 million to congressional candidates - an increase of 47 percent over the comparable period from the last election cycle.

* The Republican Party fund-raising machinery continues to hum right along. GOP party committees have raised $76 million this year; their Democratic counterparts have hauled in $13.3 million.

* Many Democratic candidates, however, are not exactly paupers. In key Senate races in Iowa, West Virginia, and Tennessee, Democrats have amassed more cash than their GOP opponents. Budget Committee chairman James R. Jones (D) of Oklahoma may have the richest campaign treasure hoard in the House.

Campaign finance is today a complicated business. Congressional candidates and parties are required to file inch-thick forms with the FEC, so detailed they list the $35 spent on dry-cleaning bills, or the $100 received from that schoolteacher in Dubuque, Iowa.

No longer can rich individuals bankroll candidates, as they did during the 1800s. Politicians must scramble for funds from a myriad of sources - their parties, PACs of all persuasions (there is even a federally registered Nazi PAC) , and hundreds of small doners.

In fact, the only people allowed to spend as much money as they want directly on a campaign are the candidates themselves. Thus John D. Rockefeller, say experts, could well spend more money on politics this year than any other US citizen, as he attempts to leap from the West Virginia governor's chair to the spot vacated by retiring Sen. Jennings Randolph (D).

But the total cost of the West Virginia Senate race probably won't measure up to that of the North Carolina battle of the titans. Political observers are watching the contest between incumbent North Carolina Senator Helms (R) and Gov. James B. Hunt Jr. (D) with fascination, as they would any spectacle of excess.

''Hunt-Helms is clearly going to be the most expensive race in the country, outside of the presidental one,'' says Herbert Alexander, director of the Citizens Research Foundation, an organization that studies campaigns.

As of June 30, Helms had out-fund-raised Hunt, $9 million to $5 million. But Helms relies on expensive direct mail for his cash, Mr. Alexander says. He must spend a lot of money to raise money, and many of his donors don't live in his state. ''He may have $9 million, but that doesn't mean it's going to be spent in North Carolina, against Hunt,'' he says.

The big boys of the PAC world - ideological groups such as the National Conservative Political Action Committee (NCPAC) also rely on direct mail. Typically, only about one-quarter of the cash they raise is spent on politics, estimates Margaret Latus, a political science professor at Holy Cross. The rest goes for operational expenses.

But they are so big that even one-quarter of their funds is a lot of money. NCPAC, largest PAC by almost any measure, raised $10 million in the 18 months ending June 30. So far, the group has made $2.9 million in ''independent expenditures,'' advertising that promotes President Reagan but is produced without any official GOP involvement.

''We plan to spend $12 million in behalf of Reagan,'' says Craig Shirley, NCPAC director of communications.

PACs continue to proliferate. There are now more than 3,500 registered with the FEC, and the value of their donations to congressional candidates is still rising at a rapid clip. Contrary to their public image, PACs may not be primarily Republican animals. Of the $50 million PACs have donated to candidates this election cycle, 60 percent has gone to Democrats, according to a Wall Street Journal analysis.

The Republicans, however, have the advantage of a party with turbo-charged fund-raising machinery. Between April and June of this year, GOP committees sucked in $16 for each $1 raised by the Democrats. Though much of their cash goes for operational expenses, Republicans spend lavishly on their candidates.

''They have already raised enough money to give the maximum (allowed by law) to all close congressional races,'' complains Rep. Tony Coelho (D) of California , chairman of the Democratic Congressional Campaign Committee.

Republican candidates, however, are not on the whole wildly richer than their Democratic counterparts. In 1982, for instance, the mean expenditure for Democratic House candidates was $213,000; for Republicans, the figure was $245, 000.

In Iowa's Senate race, Rep. Tom Harkin (D) has amassed $1.4 million, while incumbent Sen. Roger W. Jepsen (R) has $1.2 million. In West Virginia, Democrat Jay Rockefeller can outspend any opponent; in Tennessee, Rep. Albert Gore Jr. (D) has $2 million to spend on his Senate race, far more than his Republican opponent.