Support for Democratic candidates began firming up in House and Senate races around the country last week as strays from the party's natural constituency appeared to be coming home from a two-year toot with Reaganomics.

Pollsters for both parties agree that the hard-core Democratic vote in this midterm election has started to jell -- most dramatically in races in swing states where the Democrat is waging an aggressive, negative media campaign that successfully weds the Republican candidate to the administration's economic policies.

The pollsters are divided as to whether this movement portends a broader swing that will produce landslide Democratic victories on Nov. 2, or whether it is a more limited phenomenon.

Peter D. Hart, a top Democratic pollster, joined the ranks of his party's bulls last week, predicting that the independent and undecided vote will break heavily Democratic in the final days.

"Any Republican with a lead of less than 10 points right now ought to be on somebody's watch list," says Hart, who believes that the troubled economy will overshadow local and personality issues in the 435 House and 33 Senate races. The Democrats are trying to make the campaigns a "big ticket" referendum on Reaganomics and the economy, which they consider the Republicans' point of maximum vulnerability.

Robert Teeter, a top GOP pollster, acknowledges the Democratic trend but doesn't expect any dams to burst.

"The Democrats would like to think they're on a roll, but that's not it," said Teeter. "It's a normalization. The voters they've been getting this month they were going to pick up, sooner or later. They're their voters.

"In a lot of these races, the Republican lead was just too big at the start. You knew it was going to come down."

Moreover, GOP strategists think the Democratic gains, particularly in Senate races that have tightened up unexpectedly such as Missouri and New Jersey, are the product of negative advertising attacks that the Republican candidates still have time to counter.

"The lesson of this year is that if you're a Republican and you're going to be a nice guy, you're going to lose," said one top GOP staffer.

Tracking surveys in the New Jersey Senate race show that Republican Rep. Millicent Fenwick's once comfortable lead over businessman Frank Lautenberg has dwindled to almost nothing.

Republicans atttribute the slide to a "window of vulnerability" created by two weeks of heavy, unanswered negative ads by Lautenberg, and they say that Fenwick, whose own commercials began to strike back last week, still has time to rebound.

GOP strategists say that Sen. John C. Danforth (R-Mo.) finds himself in a nearly identical position against a hard-charging challenge from state Sen. Harriett Woods.

In Minnesota, Sen. David Durenberger (R) suffered a similar free fall until two weeks ago, when his ads returned fire against department store heir Mark Dayton. Tracking surveys, Republicans say, show the Durenberger lead opening back up.

At all levels, the midterm election is turning into one of the most negative in recent memory. Nationally, it is a dour and downbeat debate about the failures of the present versus the failures of the past; locally, it is a seemingly endless procession of personal jibes, insults and character assassinations.

In one Senate campaign that appears to be going well for the Republicans -- the California race between Gov. Edmund G. (Jerry) Brown Jr. (D) and San Diego Mayor Pete Wilson -- Brown's record and idiosyncrasies, which Wilson is now effectively exploiting, seem to be outweighing the economic issues.

But for the most part, Democrats believe they are getting the better of this nationwide finger-pointing contest.

Ann Lewis, political director of the Democratic National Committee, says her party's congressional candidates have gained an average of 10 points this fall in individual candidate surveys.

Party leaders say that their 19 incumbent senators, with the possible exceptions of Howard Cannon of Nevada and John Melcher of Montana, are not at risk, and that their 11 Senate challengers and three open-seat candidates are almost all closing gaps.

Morever, the 36 governors' races appear to be an even richer lode than the party first thought. Democrats have leads of 15 points or more in four midwestern states (Ohio, Michigan, Minnesota, Wisconsin) where Republican governors retired, and Democrat Roxanne Conlin is making a surprisingly stiff challenge in a fifth such state, Iowa. The party expects to pick up a net of five to 10 governorships.

These signs of Democratic momentum have not been lost on the White House, which began trying last week to lower expectations.

At midweek, David Gergen, director of communications for the White House, said that any Republican losses "on the sunny side of 20" in the House would have to be considered a major GOP victory.

In the elections since World War II that most closely resemble this one -- 1954, 1962, 1970 and 1978 midterm elections two years after the White House changed parties -- the average House loss by the "in" party was 12.5 seats.

The White House plans for the final week of the campaign appear essentially defensive.

The president's travel schedule calls for him to stump in North Carolina Tuesday on behalf of congressional candidates, then swing through the equally friendly environs of the Rocky Mountains states, Montana, Nevada and New Mexico at week's end on behalf of Senate and gubernatorial candidates.

The key is where he isn't going: big states with close and high visibility Senate or governor races, such as New York, California, Virginia. In each instance, local GOP officials have sent the White House a thanks-but-no-thanks signal and the president appears happy to avoid thrusting himself into possibly losing efforts.

The president's role in this campaign is at the heart of a dilemma the "in" party usually confronts in a midterm election, especially when its policies have not yet worked as hoped. They have the potent campaign weapons of the presidency and money -- in this case, Reagan's persuasive powers and a GOP bankroll for a $15 million media blitz. But both weapons have the effect of nationalizing the election, which is what the Democrats want -- to make it a referendum on broad economic issues, rather than a collection of 500 local contests.

Democrats draw a contrast with 1980, when the GOP's late, heavy media buy had a more upbeat theme, played better to the mood of the voters, and helped to swing many late-deciding voters into the Republican column.

This year, the most hopeful reading from the Republican standpoint comes from gazing at the campaigns in "micro" rather than "macro" perspective.

Up close, the GOP advantage in individual races in money, polling information, direct mail, media, and get-out-the vote techniques appears formidable. The skill of the GOP's 52-member House freshman class in making use of their incumbency is impressive.

"The people who think there is going to be a big Democratic sweep are the people who can't name the candidates in more than five races," says Bernadette Budde, political director of the Business-Industry Political Action Committee.