The national Republican Party is throwing the full weight of its money, manpower and advertising into a final-stretch drive to translate President Reagan's commanding lead in the presidential race into the kind of congressional sweep the party won four years ago.

This coattail-stretching operation will play out over the next five days in the scheduling of campaign stops by Reagan and Vice President Bush, in direct mailings, in advertising and in a general shift of message that could strengthen Reagan's hand in Congress during a second term.

"In the presidential election people have mostly made up their minds, but they have put off deciding in the congressional vote," said Joseph Gaylord, executive director of the Republican campaign committee in the House. "In the last two weeks, 30 percent are still deciding. In the last day or weekend, 10 percent are still deciding. That's why the Reagan-congressional election connection is very helpful."

As things stand now, it is unclear whether the election would give Reagan the kind of Congress he had after his 1980 victory, when Republicans won control of the Senate and put together a bipartisan conservative majority in the House. That coalition gave the new president a string of stunning legislative victories in 1981.

Democrats and Republicans agree that an extra push in the campaign's last days could make the difference in Senate and House races.

House Democrats expect the GOP to pick up no more than 10 seats. But Martin Franks, executive director of the Democratic Congressional Campaign Committee, noted that "if people vote in House races with Reagan in mind, we may have a problem."

In the Senate, where Republicans have a 55-to-45 seat edge, they are expected to lose a seat from Tennessee and are running behind in Iowa. They are in a virtual standoff with the Democrats in Illinois and North Carolina and could pick up a seat from Massachusetts, with long-shot possibilities in West Virginia and perhaps other strongly pro-Reagan states.

While even Democrats concede that the Republicans will keep control of the Senate, strategists for both parties say a GOP loss of one or two seats looks most likely now.

In the House, where Democrats regained an effective majority in 1982 and now have a 99-vote edge, strong challenges are under way in about 65 of the 435 districts, including some of the 27 open seats, some of the 60 seats held by freshmen and a few of those held by vulnerable Democratic veterans. Republicans contend that signs point to a gain of two dozen or more seats, which they say is enough to create a "working majority" of Republicans and conservative Democrats again in the House. Gaylord says their polls show more voters than ever identifying themselves as Republicans or expressing confidence in the party. And, he said, voters for the first time split almost evenly over which party they will support for the House. Democrats say voters are more likely to base their decision on individuals rather than parties.

Among the Republican "coattail" initiatives is a series of advertisements targeted at traditionally Democratic young voters. In one, a son and father are in their front yard, on the way to vote. The son asks if they are "still voting Democrat" even though they've done well under the Reagan administration and House Democrats are "promising higher taxes." The father replies, "When you close that curtain, who knows?"

Campaign officials said the ads will air in three contests in Texas, as well as in Alabama, Arkansas, Arizona, Michigan, Oklahoma and Utah.

Reagan has written letters for 60 Republican candidates to be used in mass mailings and has filmed commercials for candidates in 20 districts that the GOP thinks it can take from the Democrats.

In addition, the Republican National Committee is sending out 50,000 letters in 35 targeted districts appealing to voters to vote the whole Republican ticket.

In the Senate races, prospects for the kind of Reagan "coattails" sweep that ended a quarter-century of Democratic control four years ago are clouded by the fact that Republicans are defending nearly twice as many seats as the Democrats, a reversal of 1980.

There is also historical precedent for a president to win a second term in a landslide while losing ground in the Senate. In 1972, President Richard M. Nixon won a crushing majority over Democrat George McGovern even as the Republicans dropped two seats in the Senate.

But Nixon did not make the kind of "coattails" push Reagan is making, and there are enough tight races, as well as theoretically winnable races in heavily pro-Reagan states, to suggest the opportunity for a better-than-expected showing for Senate Republicans.

As of yesterday, Mitchell E. Daniels Jr., executive director of the National Republican Senatorial Committee, was saying the Republicans could fall as low as 51 seats or go as high as 56, a net gain of one. But the "most likely" outcome was "somewhere in between," he added, indicating a loss of one or two seats.

Democrats said they expect to pick up two seats if the presidential race remains as it appears now and three or four if Walter F. Mondale gains substantial ground between now and Tuesday. But J. Brian Atwood, Daniels' counterpart on the Democratic Senatorial Campaign Committee, conceded that the Republicans are virtually assured of retaining control.

A key question is whether the Democrats do enough damage to the Republicans this year to set themselves up to reclaim control of the Senate in 1986, when Republicans will have to defend even more of their shakiest seats.

Another is the impact of possible Republican losses on the conservative-moderate balance of the party in the Senate and its degree of discipline in a second Reagan administration. Already lost through retirement are Majority Leader Howard H. Baker Jr. (R-Tenn.) and Armed Services Committee Chairman John G. Tower (R-Tex.), two of the staunchest and shrewdest administration loyalists, and most of the vulnerable incumbents are conservatives.

At least one GOP seat will almost certainly end up Democratic. Even Republicans concede that Rep. Albert Gore Jr. (D-Tenn.) is the prohibitive favorite over Victor Ashe to succeed Baker.

Another Republican seat is in serious jeopardy, with both parties rating Iowa Sen. Roger W. Jepsen the most vulnerable incumbent.

A Democratic poll last week showed Rep. Tom Harkin (D-Iowa) leading Jepsen by 14 points, a "sure bet" in Atwood's view. Republicans say their polling shows Harkin narrowly ahead but contend that Jepsen is recovering from a slump a couple of weeks ago, when GOP strategists had virtually given up on him.

Even according to Republicans, Harkin scored a direct hit in television ads depicting the conservative Jepsen as "Red-Ink Roger" for a series of votes to increase government spending. But Jepsen reportedly has been helped by televised boosts from liberal Rep. Jim Leach (R-Iowa) and conservative Sen. Charles E. Grassley (R-Iowa), who debunks Harkin's contention that it is Jepsen, not Harkin, who is the odd man out in the Iowa delegation. Reagan is also scheduled to visit Iowa again before the elections.

Rated as virtual tossups are races for Republican-held seats in North Carolina and Illinois.

In the bitter North Carolina race, a poll by the Charlotte Observer showed Republican Sen. Jesse Helms leading Democratic Gov. James B. Hunt, 47 to 43 percent. More recent internal polling by both parties shows Helms leading by a narrower margin, just enough to suggest a dead heat if the Democratic voter-turnout operation is as good as the party claims.

In the almost-as-strident Illinois contest, Sen. Charles H. Percy has been teetering between a draw and a narrow lead in most polls. Republican campaign sources said their most recent information indicates a dead heat, and Democrats claim that the momentum in the race is going their way. Percy jumped early and hard on Reagan's coattails, and Reagan will be in Illinois twice before Tuesday.

Potentially as close is the race for a Democratic-held seat in Massachusetts, where Republican businessman Raymond Shamie appears to be within striking distance of Democratic Lt. Gov. John F. Kerry for the seat now held by retiring Sen. Paul E. Tsongas. Shamie is benefiting from a strong showing for Reagan in this once staunchly Democratic state, although Democrats say he has been damaged by highly publicized past links to the John Birch Society. Both Reagan and Mondale will be in Massachusetts before Election Day.

There are few other states where Democratic-held seats are in jeopardy. One is West Virginia, where Gov. John D. (Jay) Rockefeller IV is running ahead of GOP businessman John R. Raese, though Rockefeller's lack of popularity, and Reagan's strength in a state with straight-ticket voting habits, encourage Republicans to hope for an upset. The race is for the seat of retiring Sen. Jennings Randolph (D).

A long shot for the Republicans is Nebraska, where Republican Nancy Hoch is running well behind Sen. J. James Exon (D) but could be buoyed by Reagan, who is about 40 points ahead of Mondale in the state.

In at least four other states, Republicans are feeling some heat, although all are running ahead of their Democratic opponents.

In Texas, where Democrats had hoped to stage a comeback with the retirement of Tower, Rep. Phil Gramm, a conservative Democrat-turned-Republican, appears comfortably ahead of state Sen. Lloyd Doggett, although Democrats claim that Doggett can still win.

In Mississippi, potentially the closest of the four, Sen. Thad Cochran (R) has consistently led former governor William F. Winter. Democrats say Winter is closing the gap, but, so far at least, Cochran continues to get a substantial chunk of the black vote.

In Minnesota, Sen. Rudy Boschwitz (R) remains well ahead of Democratic Secretary of State Joan Growe, but a big home-state showing for Mondale could help her.

New Hampshire Sen. Gordon J. Humphrey (R) got squeezed recently by Rep. Norman E. D'Amours (D) but remains ahead.

Republicans contend that they have a crack at Sen. Walter D. Huddleston (D-Ky.) with Jefferson County Judge Mitch McConnell, but Democrats contend that Huddleston has a commanding lead.