There’s a sea change going on in the market for American painting. The field has traditionally been dominated by austere portraits of Founding Fathers, impressioniststyle landscapes, and expansive seascapes. Now, pictures by Norman Rockwell and Andrew Wyeth, once sniffed at as mere illustrators, may spike the American paintings sales, which take place tomorrow at Bonhams and Doyle New York, at Sotheby’s Wednesday, and at Christie’s Thursday.

A decade ago, works by Rockwell and Mr. Wyeth barely fetched $1 million. However, “now there is an understanding that their availability is diminishing,” the head of American paintings at Christie’s, Eric Wilding, said. That dwindling supply means higher prices.

Sotheby’s is betting that Rockwell’s original cover painting for the September 15, 1945, issue of the Saturday Evening Post, “Home on Leave (Sailor in Hammock),” will sell for between $2 million and $3 million.

With the sailor snoozing in the shade and his pet dog reclining on his knees, the picture celebrates the country’s newly won peace. The fact that the Rockwell is selling just in time for Memorial Day may boost its price. “Particularly patriotic pictures like that resonate more and fare better than ever before,” the head of American paintings at Sotheby’s, Peter Rathbone, said.

Rockwell’s Americana has been topping the price charts recently. Last November, Sotheby’s chalked up $15.42 million for the artist’s “Breaking Home Ties” (1954), also a Saturday Evening Post cover, which had been estimated between $4 million and $6 million. That painting, depicting a gawky young man and his father sitting on a running board, eclipsed the prior record of $9.2 million set for the artist’s “Homecoming Marine,” which sold at Sotheby’s in May 2006.

Doyle is gunning that the patriotic fare of Thomas Nast, considered the father of American political cartoons, will do well at tomorrow’s sale of European and American art. Nast’s oil painting “The Halt (The Drink of Water)” (1864), depicting a Union soldier taking a cup of water, bears a bullish $60,000–$80,000 estimate. Nast painted the oil while working as a Civil War artist and correspondent for Harper’s Weekly. Nast’s record price is only $20,900.

Spurring interest for Mr. Wyeth are traveling museum exhibitions such as “Andrew Wyeth Watercolors and Drawings: Selections From the Marunuma Art Park Collection, Japan,” which recently closed at the Cincinnati Art Museum. The exhibit opens at the Tulsa, Okla., Museum of the Americas on June 1. Closer to home, the Norman Rockwell Museum in Stockbridge, Mass., is touting three shows devoted to the artist, including “Norman Rockwell’s 323 Saturday Evening Post Covers.”

With such widening exposure, Christie’s expects Mr. Wyeth’s 1973 tempera “Ericksons” to reach between $4 million and $6 million, the highest estimate ever for any of the artist’s work. It depicts Mr. Wyeth’s Maine neighbor sitting in a kitchen with a wood stove, worn linoleum flooring, and bare walls. “Wyeth captures an authentic slice of America that is disappearing,” Mr. Wilding said. He estimates that more than 50 clients are interested in the painting and 12 will bid.

Such a large clutch of potential bidders is relatively new to the Wyeth market, but over the past five years there has been a small core of collectors who are passionate about his work. “The collectors have been a silent majority who have a lower profile and are perhaps more traditional in taste,” said an Upper East Side dealer, Warren Adelson, who was instrumental in selling a cache of Mr. Wyeth’s pictures of his neighbor Helga last year. He said he believes that “Ericksons” is greatly undervalued. “Compared to a Hopper for more than $20 million, the tempera is a bargain,” he said.

There are other Wyeth pictures, including watercolors, expected to go for lesser prices. Christie’s is offering “Fog and White Dory” (1941), portraying a Maine fisherman carrying lobster traps, for an estimated $40,000–$60,000. In 1987, it sold for $20,900 at Sotheby’s, proving that some corners of the American painting market are moving slowly.

Pictures by other members of the Wyeth family are pegged with relatively low estimates this week. Bonhams is offering N.C. Wyeth’s (1882–1945) oil painting “Louise Loved To Climb” (1907), an illustration of a woman in Victorian dress, estimated between $100,000 and $150,000.

But there’s another factor driving the Andrew Wyeth market in particular. “At close to 90 years old, Wyeth is coming to the end of his career,” Mr. Adelson said. “It seems inevitable that people will wake up and smell the roses about this guy.”