Conservative organizations across the country are spending at least $18 million on behalf of President Reagan this election year in what are called independent expenditures that circumvent federal spending limits, campaign spending documents and other sources show.

Their efforts have been given official-sounding titles such as Voters for Reagan, Americans for Reagan, Women for Reagan, Americans to Re-Elect President Reagan, Citizens for Reagan in '84, American Heroes for Reagan, Taxpayers for Reagan, Americans for Reagan in '84 and Christians for Reagan.

All are projects of political action committees that share one primary goal: keeping Reagan in the White House for four more years.

According to their reports to the Federal Election Commission, they often patronize the same vendors and consultants. Because some of those same vendors also do business with the Reagan-Bush campaign or other national GOP campaign organizations, some aides to Democratic presidential nominee Walter F. Mondale have argued that the conservative PACs and the Reagan/Republican committees have illegally coordinated their campaigns.

Republican and conservative PAC spokesmen scoff at the complaint and note that the Mondale campaign is under investigation by the FEC on the same grounds, in connection with the supposedly independent Mondale delegate committees that helped him win some crucial primaries last spring.

Said Reagan-Bush campaign spokesman John Buckley: "We have played Caesar's wife with regard to any impropriety. Were any complaint to come before the FEC, we would have absolutely no problem facing it."

Frequently the products of the various pro-Reagan PACs look very much alike, especially the file-folder mailings stuffed with bumper stickers, postcards and pictures of Reagan.

Officials of the PACs, however, say it is simply a small world when it comes to professional fund-raising, direct mail services and the like.

"It's probably that way on the other side, too," said Craig Shirley of the National Conservative Political Action Committee (NCPAC). "When you find somebody who does good work at a fair price and they share the same ideology, that's a tough combination to beat."

The mailings can be distinctive despite the look-alike outsides. For instance, the American Security Council sent a "Peace Through Strength" folder to some 300,000 supporters recently. It included a "Russian/English Phrase Card" that it said could be "useful in the event Walter Mondale is elected President."

The card listed a number of questions in both languages such as "Where do I apply for ration cards?" and "How do I enroll my children in the Young Communist League?"

"Obviously, we would encourage our members to vote for President Reagan," ASC President John M. Fisher told a reporter. Fisher called the phrase card "one of the politest" ways of telling people that Mondale would take the country in "the wrong direction."

The ASC did not count the folder as an independent expenditure for Reagan but categorized it instead as a mailing "to members" that Fisher said was "not generally counted as being reportable" to the FEC.

Other PACs backing Reagan readily listed much of what they did this year as spending on the president's behalf. NCPAC (whose projects include American Heroes for Reagan, Women for Reagan, Americans for Reagan in '84 and Americans to Re-Elect President Reagan) says it is spending some $10 million on positive efforts to reelect Reagan and $2 million against the Democratic ticket.

Officials of RuffPAC, headed by financial newsletter author-publisher Howard J. Ruff, say they are spending some $2.4 million for Reagan under the banner of "Voters for Reagan," not to mention such ancillary activities as radio ads promoting Reagan and congressional candidates in the same breath. One of the 30-second spots, in behalf of Robert Dornan, Republican seeking to unseat Rep. Jerry M. Patterson (D-Calif.), is accompanied by a heartbeat and goes like this:

"1976. Carter. Mondale. Double digit inflation. Crippling taxes . . . . Hostages in Iran . . . . Ridicule. Weakness. Carter. Mondale. Congressman Jerry Patterson . . . . 1980. Reagan. Bush . . . . Growth up. Rescue in Grenada. Respect. Pride . . . . President Reagan . . . . Bob Dornan."

The Fund for a Conservative Majority conducted fund-raising efforts under the aegis of Citizens for Reagan in '84 and says it is spending close to $2.5 million, primarily on a series of TV spots in 11 targeted states.

Still other pro-Reagan efforts include the Christians for Reagan project of the Christian Voice Moral Government Fund, with television spots featuring a former Miss America attacking Geraldine A. Ferraro. The fund reported pro-Reagan spending of $251,000 as of Sept. 30, but its national director, Matthew Smyth, estimated that as much as $1 million would be devoted to the elect-Reagan effort, mainly in the South.

The National Congressional Club, part of the so-called Jesse Helms network, has reported spending $759,000 on independent efforts for Reagan in 1983-84. In California, spokesmen for Howard Jarvis' Taxpayers for Reagan project said it was sponsoring radio ads for Reagan all across the country; they offered no dollar figures.

Evidently concerned by the heavy pro-Reagan spending in addition to the $40.4 million in public funds allocated by law to each candidate, some Mondale campaign officials composed what appears to be a draft complaint to the FEC. But sources said higher-ups in the campaign decided against filing it. The language relies heavily on a 1980 FEC advisory opinion which said that use of the same consultants and vendors by candidates and by organizations involved in independent expenditures "may compromise the independent nature of the expenditures . . . . "

The memo points out that a number of vendors for NCPAC, Ruffpac, the Fund for a Conservative Majority and the National Congressional Club also perform work for the Reagan-Bush campaign, the Republican National Committee, the National Republican Senatorial Committee and the National Republican Congressional Commiittee.

For instance, FEC records confirm, Bruce Eberle & Associates of Vienna and allied firms such as Omni Direct Mail and the Omega List Co. have provided services to NCPAC, the Congressional Club, the Fund for a Conservative Majority, the Republican National Committee and the Republican Senatorial Committee.

Ann Stone & Associates of Alexandria and an allied firm, Capstone Lists, have done consulting work or provided lists for NCPAC, FCM, the Congressional Club, the Republican National Committee and the Republican Senatorial Committee.

A Bethesda mailing service firm did work this summer for both NCPAC and the Reagan-Bush committee. So did a Newington, Va., envelope company and a Cheverly, Md., computer services firm.

Suggestions that all this might signify some conspiratorial link were dismissed by Eberle as a lot of "bunk." He said similar allegations against NCPAC and the Reagan-Bush campaign had been considered and dismissed by the FEC earlier this year.

"If they get a list from us, so what?" Eberle said. "They get their electricity from the electric company and their telephone service from the telephone company. Does that mean they're in a conspiracy? I think it's an argument for a losing cause."

So did Ann Stone and her husband, Roger Stone. He is eastern coordinator for the Reagan-Bush campaign and his firm, Black, Manafort & Stone, does consulting work for Reagan-Bush.

"We are very aware of our unusual situation and therefore take extra pains to avoid any problems," she said. "We take our politics to bed. We don't take our business to bed."

"We're married," her husband said. "I guess that's the allegation and I plead guilty to it." He said he thought it especially dubious to equate work for the Republican congressional campaign committees with work for the Reagan-Bush campaign.

"They're different committees with different missions and different budgets," he said. "The weak reed is the 'Reagan/Republican committees' link."