As the Senate prepares for today's showdown with President Reagan over his plan to sell missiles to Saudi Arabia, the ghost of Republican ex-senator Charles Percy of Illinois and the specter of Big Money both haunt the Capitol.

sk,3 In 1984, Percy, then chairman of the Senate Foreign Relations Committee, was defeated for reelection by just 89,000 out of nearly five million votes cast. Percy described as "my biggest problem in the campaign" a 37-year-old California businessman who spent $1.2 million for mailings, billboards and commercials that were relentlessly critical of the candidate. Senate colleagues who generally held Percy in more respect than affection can personally identify with that kind of negative bombardment.

sk,3 Now in the midst of the fight over the administration's proposal to sell arms to the Saudis, that anti-Percy California businessman has been in Washington, where audiences have been arranged with Republican senators supporting the president on the issue. As The Post's Edward Walsh reported, the script for these meetings was blunt: the arranger and interlocutor was Sen. Rudy Boschwitz (R-Minn.), who presented the California businessman, Michael J. Goland. With all the subtlety of a knee to the groin, Boschwitz, according to White House and Senate Republican sources, told the senators that this was the same guy who had spent all that money to beat Percy because he disliked Percy's lukewarm support of Israel.

sk Consistent with Boschwitz's delicate touch, Goland reportedly told the GOP senators to consider the possibility of one of those Saudi missiles falling into the hands of a terrorist who would use it to shoot down a civilian airliner, killing Americans aboard. Then, reports indicate, Goland asked the Republican senators to imagine the political repercussions of a TV commercial of that airplane wreckage -- yes, the one with the dead Americans -- reminding home-state voters that the senator had voted for such arms sales. This was no idle threat. Add to that the professional reference supplied by Boschwitz, who has not concealed his ambition to be the next chairman of the Republican Senate Campaign Committee, and you have a formula for intimidation by Big Money.

To their credit, Republicans Sens. Dan Evans of Washington and Phil Gramm of Texas were not intimidated. But they were among only 22 senators who supported the president on the arms sale while 73 opposed it.

Undoubtedly some -- especially those with access to it -- would welcome a return to the pre-1974 days when Big Money spoke with authority and when candidates listened with respect and deference. That was a time when such raw threats were common in politics. It was only 14 years ago that we had a presidential campaign in which $117 million was contributed by just 534 individuals, which in turn produced a national scandal. After that, by law we sought to discipline and to restrain the influence and use of money in our campaigns. And, to a remarkable degree, we have succeeded.

While an individual citizen's contribution to a federal candidate's campaign is statutorily limited to $1,000, expenditures that are independent of any candidate's campaign cannot be legally limited. That joint private appearance by Goland and Boschwitz, while suggesting a scene from "The Goldfather," raises serious questions about the supposed independence of such unlimited spending.