After the expenditure of more than $300 million and the bombardment of the public with hundreds of biting, negative advertisements, today's midterm elections seem unlikely to be the national referendum many anticipated, with the likelihood of disappointments for both parties.

The Republicans' once-gauzy dreams of capturing control of the House of Representatives and their solid hopes of increasing their majority in the Senate on their way to becoming the majority party have given way to the reality of holding down their losses.

For the Democrats, the hoped-for referendum on Reaganomics apparently has turned out to be considerably less than that, as the potential landslide that appeared to be developing in mid-October evaporated in the face of a huge influx of money into GOP campaigns and an advertising blitz encouraging voters to "stay the course."

For the public, whose mood has been difficult to read this year, there may be a measure of relief that the campaign is over and that politicians will be forced to stop blaming each other for current problems and return to the task of putting the battered economy back on track.

Party strategists said yesterday they were anticipating only modest Democratic gains in the House, continued Republican control of the Senate and increased Democratic strength in the statehouses.

Weekend polling showed movement toward Republican Senate, congressional and gubernatorial candidates in about a dozen states.

Final estimates by party officials were that the Democrats would gain 15 to 20 seats in the House.

"I think that we got badly outspent and that will show," said Ann Lewis, political director of the Democratic National Committee. "That stopped our gain."

The estimates of House gains conformed closely to the final Harris survey on the midterm elections, which showed voters preferring Democrats to Republicans in congressional contests by 51 to 42 percent. That translates to a gain of 12 to 20 seats, the survey said.

In contrast, the final Gallup Poll, released last week, gave the Democrats a 55 to 45 percent lead over the Republicans. Those percentages cannot be translated directly into party gains, the Gallup organization said, but predicted the Democrats would "add substantially" to their House majority.

In 1978, when they captured 54.4 percent of the popular vote, Democrats won 277 seats. Today they hold 241 seats, with two other Democratic seats vacant because of a death and a resignation.

For television watchers, there are a number of early evening benchmarks. Republicans say it will be a good night for them if Sen. Lowell P. Weicker Jr. of Connecticut wins reelection and state Sen. Nancy Johnson wins the open House district in the state.

They will be watching two other Senate races for signs of unexpected strength: Virginia, where Republican Rep. Paul S. Trible Jr. faces Democrat Richard J. Davis, and Maine, where Rep. David Emery is challenging Democratic Sen. George J. Mitchell.

Republicans also will closely watch early returns from Alabama, where Montgomery Mayor Emory Folmar (R) is trying to stop George C. Wallace's comeback in the governor's race. And if Republican Rep. Christopher H. Smith survives his reelection fight against state Sen. Joseph Merlino in New Jersey, the Republicans will count Tuesday as an excellent night.

An early evening Democratic sweep might be foreshadowed by the defeat of Republican Sen. Robert T. Stafford by James Guest in Vermont or the defeat of Senate hopeful Republican Rep. Millicent Fenwick in New Jersey by businessman Frank Lautenberg.

In early reporting House races, the Democrats will know they are heading toward a big night if Marcy Kaptur defeats Republican Rep. Ed Weber in Toledo, if Rep. Joel Deckard loses and Rep. Philip R. Sharp wins in Indiana. And a real wild card would be the defeat of Republican Rep. Gene Snyder by state Rep. Terry Mann in Kentucky.

Democrats yesterday believed they were poised to capture several Senate seats in today's voting, but Republican strategists said many of their candidates had bounced back from potential losses and that Democratic gains in the Senate would be more limited.

In governor's races, the Democrats were expected to win five or six seats. But on election eve, many of those races were considered too close to call.

In Senate races, Republicans said that weekend tracking showed they were likely to hold onto seats in Minnesota and Rhode Island, which were in doubt a week ago.

In Minnesota, Democrat Mark Dayton had gained ground on Sen. David F. Durenberger, but a poll by The St. Paul Dispatch and station WCCO-TV released yesterday, showed Durenberger leading 49 to 38 percent.

Republicans said they also saw a possible gain in Nevada, where GOP challenger Chic Hecht was apparently boosted in his race against Sen. Howard W. Cannon by a visit from President Reagan last week.

The most endangered Republican, according to GOP officials, was New Mexico Sen. Harrison H. Schmitt, whose campaign against Democratic Attorney General Jeff Bingaman has been sliding since a series of negative ads backfired 10 days ago.

In Missouri, some Republicans believe that a new round of negative ads aimed at Democratic state Sen. Harriett Woods helped Sen. John C. Danforth stop his slide, but they were still nervous about Danforth's chances.

In New Jersey, Fenwick, who had slipped last week in her Senate race against Lautenberg, was in position to win the election today, but was far from comfortable.

"Right now we'll win it, but it's close," said one Republican strategist.

An Eagleton Institute Poll released yesterday showed the race dead even, with Fenwick topping Lautenberg by 46 to 45 percent. Another poll by The Record of Hackensack, which was conducted between Friday and Sunday, showed that among likely voters, Fenwick had gained strength and led 53 to 47 percent.

Democrats said yesterday they still had a chance to defeat Danforth in Missouri -- a view shared by some Republicans -- John H. Chafee in Rhode Island, Weicker in Connecticut, and listed Schmitt in New Mexico as theirs to target.

They also held out some hope of seeing California Edmund G. (Jerry) Brown Jr. defeat Pete Wilson in the California Senate race, but Republicans said Wilson was comfortably ahead.

In governor's races, Democrats were expected to add seats in Ohio, Wisconsin and Minnesota. But in Michigan, where Democrat Jim Blanchard once appeared an easy winner over Republican Richard Headlee, late polls showed that the race was now close enough to give Headlee a chance.

Democrats said they expected to hold the governorships of New York and California. A late surge by Republican Attorney George Deukmejian put him into position to overtake Los Angeles Mayor Tom Bradley in California, but tracking on the race indicated that Bradley still led by several points yesterday.

In Alabama, Wallace remained a slight favorite over Folmar, but Republicans said the race was a tossup.

In Texas, Republicans said Gov. Bill Clements had a safe lead and would win with 52 to 54 percent of the vote, but Democrats insisted their own polling still showed Attorney General Mark White in range to score an upset.

A number of races were considered too close to call. Democrats were in danger of losing the governorships of New Hampshire, Kansas, Idaho and New Mexico. Republicans were worried about losing Alaska and Iowa.