What would it take for a BIG Democratic win in Tuesday's election?

A big Democratic win in next Tuesday's national election would be a tale of two surges.

The first occurred in mid-October, after double-digit unemployment was announced and President Reagan went on nationwide television to ask for patience. Suddenly, GOP Senate incumbents once thought safely ahead found Democratic challengers either breathing down their necks or pulling ahead.

It will take a second and stronger Democratic surge, however, to clinch victory.

This week many Republicans eeked open a little daylight in their races. It is up to Democrats to mount a final drive this weekend to pull off a major upset, such as recapturing control of the US Senate.

''The Democratic run came the week after the unemployment numbers,'' says Vincent Breglio, executive director of the National Republican Senatorial Committee. ''Our races tightened - even alarmingly. But the margins have started to come back. I'm not going to say we're not going to lose some. But we see some daylight on the lot of them.''

The shifting forces in this campaign are seen much the same way by Democrats - only they see the first surge lasting into this week. Democratic political consultant Peter D. Hart cited as an example of the Democrat's October drive one Senate race in which the GOP lead was 53-25 percent in August. Last weekend it was 45-42.

It remains to be seen whether the Democrats have already peaked, or whether they will be able to turn out their vote after the final four days of the campaign.

Moderate Republican senators seem to have borne the brunt of public dismay over the recession. Incumbent Republican Sens. John C. Danforth in Missouri, David Durenberger in Minnesota, Harrison H. Schmitt in New Mexico, and four from the Northeast - Rhode Island's John H. Chafee, Connecticut's Lowell P. Weicker Jr., Delaware's William V. Roth Jr., Vermont's Robert T. Stafford - have seen their margins erode sharply. So has New Jersey Republican candidate Millicent Fenwick, another GOP moderate, in a race with Democrat Frank Lautenberg.

Even if most of these moderates survive, their close call has already sent a message to the White House and to the remaining GOP moderates - many of them, such as majority leader Howard H. Baker Jr. and finance committee chairman Bob Dole, in Senate leadership positions.

''It sends a signal to the White House to be more pragmatic about its economic policies,'' says William McKenzie, executive director of the Ripon Society, a moderate GOP organization. Without cuts in defense and other changes to cut the ''hard costs in business failures and unemployment'' in Mr. Reagan's program to date, there will be more GOP losses in 1984 when the majority of Senate incumbents up for reelection will be Republican, he warns.

Going into the final weekend of a campaign, political observers traditionally start to hedge their bets. They cite the fact that one quarter of the electorate usually makes up its mind from the last weekend to the last moment. Turnout is another unknown. Black voters, strongly opposed to Reagan, may vote in greater numbers. So may women. National surveys of voter preferences for parties give the Democrats a sharp lead, but national surveys in 1980 did not prepare the public or the pundits for the big Reagan-Republican victory.

This is how the party professionals look at key races going into the final weekend:

Whether Democrats win Senate control may hang on the California race between Democrat Edmund G. Brown Jr. and Republican Pete Wilson. The Democrats would have to win five Republican seats and lose none to capture a Senate majority - a tough chore, even with a second Democratic last-hour drive. Mr. Wilson has been ahead in California since early summer, but the resourceful Brown has narrowed the margin to a few points and could take it.

Reagan is visiting New Mexico, Montana, Nevada, Wyoming, and Utah Oct. 28 and 29 to shore up GOP Senate prospects. ''Reagan's strength is in the central mountain states,'' explains Mr. Breglio.

''We felt all along New Mexico would be troublesome,'' Breglio says. ''Schmitt now has a six- or seven-point lead. It's a tough race. If Schmitt doesn't have an eight- or 10-point lead over (Democrat Jeff Bingaman) going into the election, it could be rough.''

In Utah, Republican incumbent Sen. Orrin Hatch will win by 55 percent to 45 percent over Salt Lake City mayor Ted Wilson, Breglio predicts. Mr. Wilson's campaign manager, Mike Graham, concedes a final surge might prove tougher to generate in Utah than in any other state.

The other Mountain State races, with potential GOP pickups, are still up for grabs. ''The lead has changed hands twice in Montana,'' Breglio says, where Republican Larry Williams is challenging incumbent Democrat John Melcher. ''In Nevada we're dead even.'' In Nevada, Republican Chic Hecht is challenging Democratic incumbent Sen. Howard Cannon.