When Republican presidential candidate Marion G. (Pat) Robertson arrived in Champaign, Ill., for a three-hour campaign stop recently, his plane was greeted by several of Champaign's finest, who escorted him into town with sirens blaring and blue lights flashing.

At the hotel where the former television evangelist spoke at a luncheon, more plainclothes officers scrutinized arriving reporters and guarded the doors, keeping a careful watch on each of the 400 or more evangelicals who came to eat cold cuts and hear the candidate make his pitch.

The tight security came as a relief to Patrick Caldwell, a retired Secret Service agent who travels with the Robertson campaign and accompanies the candidate, even on his daily jogs. Without Secret Service protection, Caldwell worries, Robertson could be vulnerable while on the campaign trail.

At another event later that evening in the northern Illinois city of Rockford, Caldwell was once again wary. At the loud rally, there were only three security guards and more than 1,000 in the audience, including a man in the front row wearing a jacket that read "American Atheist."

Robertson is one of two candidates for the 1988 Republican or Democratic presidential nominations who has requested early Secret Service protection this year, citing the number of death threats the campaign has received. Democrat Jesse L. Jackson also has asked for early protection as he did in 1984.

"In 1984, Reverend Jackson had 311 death threats, and 14 people were put in jail," said Jackson spokeswoman Pam Smith. This year, Jackson campaign officials believe, as one phrased it, that "certainly his life is at risk to some degree."

"Just this week, we got a letter from Baton Rouge, Louisiana, saying the headquarters there was going to be blown up," said Jackson spokesman Frank Watkins.

"Any presidential candidate, it's open season for lunatics out there," said Scott Hatch, a Robertson campaign spokesman.

Hatch would not specify what threats have been made against Robertson, but the campaign guards such normally public material such as Robertson's schedule closely, officials said, for security reasons.

The decision on when to provide protection for the '88 campaigns will be made by a Capitol Hill committee of five. The cost has been estimated at $15,000 a day for the unusually large field of 12 Democratic and Republican candidates.

The Hill committee is made up of the speaker of the House, the House minority leader, the Senate majority and minority leaders and one public member, who this year will be retired Supreme Court justice Lewis F. Powell Jr.

No one questions the need for protection of presidential candidates, who travel widely and are highly visible. But this year, officials are worried about the drain such a large-scale investment of personnel -- budgeted at $30 million this year -- will have on other Secret Service responsibilities.

Bob Mills, a staff member on the Senate Appropriations subcommittee that oversees Secret Service funding, said that budget writers are bracing for a possible spending freeze as a result of deficit-reduction negotiations that could curtail funds available for quadrennial needs such as candidate protection.

One suggestion under informal consideration, Mills said, is that candidates undertake some of the cost of protection themselves. The need for protection, Mills said, is important but often candidates like to use the Secret Service presence as a "status symbol."

"If we're all biting bullets, it may be something that all of the candidates ought to think about," Mills said.

The special committee has met once, and is now considering what guidelines should be devised to decide which candidates qualify and when. Early protection to a candidate can only be offered if the five-member committee or the president makes an exception.

Secret Service protection was provided in 1984 after a candidate had qualified for at least $100,000 in federal matching funds and had raised at least $1.5 million in contributions, or received at least 10 percent of the vote in two consecutive primary elections. (That year, however, Jackson was authorized for early protection.)

"It's not written in law what the definition is," a Treasury Department spokesman said. "It's up to the {committee} members."

As the members, including Republican presidential candidate Robert J. Dole (Kan.), the Senate minority leader, consider their options, it seems almost certain that the campaigns want the government to figure out the best way of paying for protection. Asking campaigns to assume the cost, Jackson aide Watkins said, would be "out of bounds."