The signs are multiplying in realty markets across the country that the United States is heading for a stronger home-sales rebound than usual for the spring months -- despite continued high-mortgage rates.

Discussions with major brokerage firms in 18 markets during the week ending Feb. 20 point to an earlier-than-normal jump in sales, listings and buyer "traffic" compared with the past two years.

Firms in several of the largest markets -- such as Chicago, San Fransisco-San Jose and Washington-Baltimore -- report sales activity is up 15 percent to 20 percent above 1980 levels, and listing are up even more.

They attribute it in part to a growing sense of realism among consumers about the long-term cost of mortgage finance. They also believe that the huge unfulfilled demand by buyers and sellers for first-time and move-up housing that developed in 1980 is beginning to spill out into the market in 1981 -- and could be a far larger force this year than most economic analysts assume.

"We're 21 percent ahead of our own projections from the end of 1980, and frankly, we're scratching our heads," says Richard Bock, senior vice president of Chicago's Quinlan and Tyson realty firm, which sold $340 million worth of homes last year. The firm racked up $16 million in new sales at its 17 North Shore and northwest suburban offices in the first two weeks of February alone, according to Bock, and appears to be heading for $32 million to $35 million for the month.

That sort of sales pace is strong for the firm in any 30-day period, but it's extraordinary for the depths of February with interest rates in the 14 percent to 15 percent ranges.

"I think there may be something very significant brewing out there" among buyers and sellers, says Bock. "A lot of people have either decided or are getting close to deciding that they might as well take the plunge now . . . because waiting another year or two isn't really going to help."

The largest-volume firm in the Baltimore-Washington market, Merrill Lynch Realty-Chris Coile, had a 30 percent gain in sales in the first two weeks of February over 1980. Its listings have jumped from $15 million to nearly $28 million in the Baltimore market alone.

Coile's general sales manager, Bill Barrett, says "there's definitely a turn in the market" that augurs well for at least the next two or three months. t"I can't tell you what's doing it -- whether it's the weather, whether it's people's sense of optimism about inflation coming down, or whether they're beginning to see 14 percent mortgage rates as acceptable when the prime rate from banks is sitting at 19 percent. But whatever it is, I can tell you that the market has more pent-up energy in it that's ready to pop this spring than a lot of people understand."

Ivan Gates, vice president of San Jose's Van Vleck Realtors, one of the biggest in the San Fransisco Bay area, attributes his firm's recent 25 percent spurt in sales and listing volume to "buyers' recognition -- which has taken awhile coming -- that they can indeed get some very good prices and financing help from sellers."

"It's still a buyers' market here," says Gates, who heads 230 sales associates in 15 offices. "But now there's movement; now we're seeing real live buyers, people who sign contracts rather than look at houses."

Brokers in markets as diverse as Miami, New Orleans, Buffalo, suburban New York-New Jersey, Phoenix, Minneapolis and Atlanta suggested that the performance of the Reagan administration in its first month in office may be a contributing factor in consumers' feeling of optimism about real estate.

Gil Kendall, a co-owner of New Orleans' All-Star Real Estate, said the Reagan administration's "no-nonsense commitment to cut the federal budget to the bone, cut taxes and try to cut inflation makes people feel like maybe there's finally somebody at the wheel up there in Washington who knows how to drive."

Joseph Klock, head of Miami's Klock Realtors, put it this way: "Don't sell the mood of the country short as a possible factor in the bounce-back we may be in the process of seeing nationally. However you define it -- whether it's Reagan or post-hostage or post-Carter -- I think the overall mood is turning upbeat. And that's got to be good for read estate.