President Reagan embarks today on a five-state western swing aimed at preserving Republican control of the Senate and heading off a late breaking Democratic surge in the region where the president remains most popular.

Discounting reports that the Senate, which the GOP captured in 1980, might fall into Democratic hands, White House spokesman Larry Speakes said such predictions were "the stuff which pipe dreams and political columns are made of."

However, other administration officials acknowledged at least the remote possibility that these Democratic dreams could come true and said that Reagan's final swing of the campaign is an effort to forestall it.

White House advisers say they believe that the effort will succeed, and Reagan said yesterday in a GOP publication that Democrats "are in for a big surprise" if they expect heavy gains in Tuesday's elections.

But as Reagan prepared for a two-day foray to Wyoming, Montana, Nevada, Utah and New Mexico, it seemed like a strategy of "Save the Base" had replaced the familiar theme of "Stay the Course."

The five states Reagan will visit are among the 13 states where he received 55 percent or more of the vote in the 1980 presidential campaign. One, Utah, was Reagan's best state, giving him 72.4 percent of the vote.

The fact that Reagan is being brought in at the last moment to aid the Senate reelection campaign of incumbent Orrin G. Hatch is itself testament to the presumed shakiness of the GOP even in this most Republican of states.

According to administration officials, the president made the final decision to go to Utah, in response to a plea from pollster Richard B. Wirthlin.

"Suppose we had the chance to go there and didn't and then lost by a point or two," Reagan was quoted as saying. "How would we feel then?"

The strategy of the Reagan end game this week illustrates both the weakness and strength of the Republican position. One GOP strategist said the tide is running for the Democrats but said the "good news" is that Republican Senate incumbents are most vulnerable in states where Reagan can help.

On the other side of the coin, Democratic pollster Peter Hart observed yesterday:

"If I had said to you a year ago that in the last two weeks of the campaign, President Reagan would not go to New York, Pennyslvania, Ohio, Michigan or California but that he will instead go to Peoria to try to save House Minority Leader Bob Michel, to Nebraska to try to save a Republican governor and to Wyoming and Utah to try save Republican senators, what would you have said about the year? You would have said that the Republicans were trying to prevent a disaster."

Hart referred to the Reagan midterm approach as "the enclave strategy," a term accepted by administration strategists, one of whom quipped, "We haven't seen a city in two months."

Vice President Bush is seeing more cities than the president but he also is pursuing the enclave strategy, campaigning for congressional candidates during the final week in such Reagan citadels as Florida, Idaho and Mississippi. Bush also will campaign for gubernatorial and Senate candidates in California, which the president will not re-enter until he takes his Thanksgiving vacation.

The White House view is that a senator in Wyoming is worth as much as a senator from any of the megastates, and that Reagan can move a much more significant proportion of votes in the sparsely populated mountain states than anywhere else.

"I don't think the president can move a state the size of New York or Michigan," said one administration official. "I think he can make the difference in Wyoming or Utah. The best place to use the president is where he'll make the difference."

Democrats and Republicans give similar assessments of Senate prospects in four of the five states Reagan will visit. Hatch in Utah and incumbent Malcolm Wallop in Wyoming are ahead, though not by secure margins. GOP Senate challengers Larry Williams in Montana and Chic Hecht in Nevada trail, with Williams having the better chance of an upset victory.

The assessments diverge in New Mexico, where some Democrats believe that Democratic Attorney General Jeff Bingaman has overtaken incumbent Republican Sen. Harrison H. Schmitt. Republicans acknowledge that Schmitt is in trouble but believe he can win.

Republicans worry that there will be a movement of voters in the final five days of the campaign similar to the movement that carried Republicans to surprise Senate victories in 1980.

GOP trackings showed a surge toward the Democrats about 10 days ago that appears to have stopped. Reagan's western swing is a hedge designed to cushion Republicans if this surge starts again.

"There were several Democratic senators who woke up on election day in 1980 to find out they weren't senators anymore," said one Republican yesterday. "We're trying to ensure that this won't happen to us next Tuesday."