A NUMBER of bemused politicians and assorted nationalists are chortling these days over what is merrily called here "the reconquest of the lost territories," or "our infiltration" of that vast swath of land from Atlantic to Pacific that the United States took from Mexico more than a century ago.

For years, droves of Mexico's poor have gone north with a tenacity that turned Mexicans into the largest minority in the American Southwest. But now, traveling beside them are Mexico's new rich, who are not sneaking through the bushes along the border to hide illegality. They go freely in big cars, airliners and private jets with leather-bound passports and billfolds full of dollars.

Mexico's fast-growing and increasingly affluent middle class is spending millions of dollars buying condominiums in Texas and Colorado and homes in Florida and California. It is looking at, or already part of, many business ventures.

No one appears to have come up with a serious overall figure for the Mexican buying spree in the American Sun Belt. An informal and incomplete Mexican government survey showed that more than half the recent real estate sales in places like Padre Island, San Diego, Vail and San Antonio have been to Mexican nationals.

Bankers here point out that well over $4 billion left Mexico in the second half of 1981. No one knows just what happens to the freely convertible pesos, but a senior government official believes "it is safe to assume that a large portion of that capital flight has gone into American property."

The big spenders include comfortable professionals and affluent industrialists and politicians, all of whom have profited from the oil-financed industrial boom.

AMERICAN MERCHANTS have reacted quickly to their growing market south of the border. U.S. supermarket chains splash their advertisements across newspapers in Mexico's northern cities, quoting prices in dollars and pesos. For the past few months, ads in papers, magazines and radio spots here in the capital have announced the attractions of owning property in Texas, California or at American ski resorts.

Worried government officials are wondering how to stop the peso flow. The total foreign debt for 1981 has reached $63 billion dollars.

In December, the government passed a law requiring realtors or advertisers of property in the United States to obtain official permits. But some local papers and radio stations still carry such ads, claiming the law is unconstitutional.

Privately, officials concede that there is little they can do. The root of the frantic buying in the southern United States lies in the present distortions of the Mexican economy.

Mexico, long a bargain for American travelers, gourmets and shoppers, is becoming more like an expensive tourist trap. "The tables have turned; the United States has become a bargain for us," said the wife of a Mexico City businessman who takes her family of five to Houston for medical checkups, and to Los Angeles and Vail for vacations.

Inflation in each of the last three years has been close to 30 percent, but markups in consumer goods and real estate prices have doubled and often tripled during that time.

In addition, the Mexican peso, at the moment worth about 26 to the dollar, is widely regarded as overvalued. Although it is officially a floating currency, the government let it slip down just 12 percent last year. "The frequent rumors of devaluation, even if unfounded, make people want to buy, buy, buy," a foreign banker believes.

THE MEXICAN shopping splurge in the United States reportedly is making its effect felt here. Despite an explosion of consumerism in this country, Mexicans reportedly buy their electronic gadgets in the United States whenever they have the chance, where they are often half the price.

Shoppers here report that stores in San Antonio and elsewhere along the U.S. side of the border offer to deliver televisions and refrigerators in Mexico--past customs--for an extra $50.

The splurge has also affected the real estate market along Mexico's coastlines, where realtors maintain that their market has not grown along with the rest of the economy.

"I'd be mad to buy in Mexico," said a government lawyer who recently bought a lakeside apartment near Houston.

"For what we spent on our fully owned apartment I could not even get a time-sharing place near a Mexican beach.

"And the interest on the mortgage in Mexico is nearly 40 percent. I'm paying less than half that in Texas."

HARDEST HIT by the overvalued peso is Mexico's own tourism industry. Experts believe Mexico is pricing itself out of the market compared with other Caribbean resorts and Hawaii. A survey conducted by the National Bank of Mexico last year showed that hotels and food cost 8 percent more in Mexico than in the United States. In 1979, Mexico was still 28 percent cheaper.

Last year, after many years of growth, tourism here dropped by 4.5 percent, according to the bank. The exodus of Mexican tourists is tipping the balance drastically. Last year, 3.9 million Mexicans took vacations abroad, an increase of 20 percent. Their spending, most of it in the United States, went up 42 percent to almost $1.5 billion.

"IT SHOWS that all the fuss about how much oil the U.S. is getting from Mexico is irrelevant," a Mexican diplomat said recently. "The two countries are closely linked in so many other ways."

The interesting question, he went on, "is which country will have more impact on the other in the long run. Whether it is U.S. technology and consumerism imported here, or whether it is part of the Mexican culture carried by millions of our emigrants. The social and political influence of Hispanics is growing in the southern United States and now the rich Mexicans are gaining economic influence." With a smile, he added, "I must say I like the idea of our buying Texas back."