The Iowa presidential campaign heads into its final week today with the uncertainty of the turnout at the Jan. 21 caucuses clouding the shaky predictions of victories for President Carter and Ronald Reagan.

"I still think Ronald Reagan's the guy to beat," said Republican State Chairman Stephen Roberts, after a poll Friday showed Reagan support skidding. "But I'd say George Bush has a fairly good chance of upsetting him."

Democratic State Chairman Ed Campbell brushed off the poll's finding of a 2-to-1 lead for Carter over Sen. Edward M. Kennedy, saying, "It's still a horse race. Polls don't translate into caucus attendance."

Historically, they have not. Compared to primaries, caucuses turn out only a fraction of the electorate -- normally just party loyalists.

The 1976 Iowa caucuses, which gave Carter his first big boost on the way to the White House, attracted only 7 percent of those eligible, 38,500 of the 549,000 registered Democrats. On the Republican side, only 20,000 of 477,000 attended.

With the heightened attention the candidates and the media have paid to the contest this year, everyone predicts that far more people will attend.

On the Republican side, the assumption -- until this week at least -- has been that a high turnout benefits Reagan, the popular choice of GOP voters; on the other hand, a low turnout figures to help George Bush because he has recruited much of the GOP establishment into his camp.

Reagan, in his handful of appearances in the state and in radio ads, has urged audiences not to fear the caucuses. "It's not just for party regulars," he says. "It's a neighborhood meeting for people who believe in Republican principles."

But a Des Moines Register poll that reported Reagan's popular support among Republicans had dropped by almost half since last month -- from 50 to 26 percent -- raised questions about that assumption.

"I doubt the (Reagan) theory that the bigger the turnout, the better for them; the new people who would come in are not Reagan sympathizers; new people are looking for new candidates," Bush's campaign chairman, George Wittgraff, said earlier last week.

For months the Republican race has been a two-tier affair. On one tier, far above the pack, has been Reagan. On the other tier have been the other six candidates jockeying for second or third place and a shot at dislodging Reagan in later primaries.

Bush has been in the state the longest and worked the hardest, spending 27 days here since last March compared to Reagan's 34 hours. And his two closest rivals in the pack, Sen. Howard H. Baker Jr. (R-Tenn.), and former Texas governor John Connally, speculate that he may upset Reagan.

"There's no question we will have done more than any other campaign" to contact voters, Bush chairman Wittgraff said, "Reagan has more support, but we'd like to think ours is more motivated, and they've certainly been contacted. We'll get out a higher proportion of our support but Reagan has to be favored."

In what was probably the most intensive political week in Hawkeye State history, the major candidates of both parties and their surrogates blitzkrieged the state last week trying to stir up support. By today, most had left their campaigns in the hands of their field organizations.

The Kennedy organization, confident of victory only weeks ago, goes into the week frustrated by the inability of its candidate to come to grips with his insulated opponent in the White House. Over drinks late one night recently, Paul Kirk, Kennedy's chief political strategist, complained about the unexpected turn of events.

"With this international thing, and no debate," he said "it's like being in the middle of one of these cornfields with your hands tied behind your back," he said.

With uncertainty hovering over events in Iran and Afghanistan, Kennedy turned his attack on Carter's partial grain embargo against the Soviet Union, a heated issue iowa. But even here, aides worried that the patriotic fervor the Carter administration has created over the issue, and Kennedy's obvious unfamiliarity with farm policies limited his effectiveness.

By contrast, morale in the Carter campaign has done a dramatic turnabout. When Scott Burnett, who normally works in the White House scheduling office, arrived here from Washington 10 days ago, he looked like a young man on his way to a hanging.

Burnett was "on vacation" -- a paid, political vacation on behalf of his boss. He had been sent to Council Bluffs, an old river town across the Missouri from Omaha, Neb. to beef up the Carter reelection effort.

Kennedy, he worried on the way from the airport, was popular and well-organized in the state. It could be devastating for the president.

That was Jan. 2. This past week he was all smiles. "We could win here, and we could win big," he said.

What happened is a case study in the use of the powers of incumbency. Since Burnett arrived he has been inundated with help from Washington. Sam Brown, director of Action and a Council Bluffs native, has paid a visit. So have Rosalynn Carter, Agriculture Secretary Bob Bergland and Labor Secretary Ray Marshall. Vice President Mondale is supposed to drop by this week, and local officials last week found that long-awaited $57,000 Housing and Urban Development grant had suddenly materialized for their community.

Six members of the "Peanut Brigade," a group of Carter supporters from Georgia, have arrived to help organization efforts. A top aide of Sen. Thomas Eagleton (D-Mo.) has moved in for a week to help. The executive director of the Kansas Democratic Party also has lent a hand. And Carter has spent several hours on the telephone talking to supporters. During one call to a group of Carterites, a 17-year-old girl mentioned she'd be visiting Washington in mid-March.

"Oh, yeah, I wish you'd come by and see me. Get a hold of Tim Kraft or Laurie Boggs and they'll set it up," Burnett recalls Carter as saying. "So now she's going to visit the Oval Office, and she's become one of our most dedicated supporters."

Among Republicans, Baker and Connally, both of whom have had trouble building organizations in the state, appeared to be making inroads urging the past week. Baker is the only major candidate to schedule a full week of campaigning here this week. Connally, who is outspending his opponents, has scheduled a television call-in show in Des Moines Thursday before a host of other candidates return to the state for a last-minute weekend swing.