ATLANTA (AP) _ Michael Dukakis has chosen in Lloyd Bentsen a millionaire running mate whose re-election warchest is bigger than that of any other senator this year.

A Texas landowner whose personal worth is between $1.4 million and $2 million, Bentsen is a well-dressed patrician who for a time drove a Mercedes Benz, one of the few lawmakers brave enough to drive an imported car while shepherding sensitive trade legislation through Congress.

He owns considerable real estate in the Rio Grande Valley and elsewhere in Texas as well as a farm in Virginia's Shenandoah Valley. His family is wealthy but Bentsen made his own fortune in business in Houston.

While his personal wealth is substantial, it is the Democratic senator's campaign fund-raising talent that is the envy of many colleagues.

Bentsen has raised more than $5 million so far for his re-election bid, should he decide to seek a fourth term while also seeking the vice presidency.

Bentsen already has spent $2 million on his campaign and ranks second among Senate candidates with money in the bank - $3.7 million as of March 31, when the last campaign finance reports were filed with the Federal Election Commission.

He has received more money from special interest political action committees than any Senate candidate seeking election this year - $1.46 million. By contrast, Dukakis refuses to accept PAC funds.

Bentsen became embroiled in a controversy last year when it was disclosed he was seeking campaign contributions of $10,000 from lobbyists in return for their membership in a ''breakfast club'' that met once a month.

After press reports surfaced about this ''Chairman's Council,'' Bentsen acknowledged in February 1987 that he had made a ''doozy'' of a mistake in setting up such a committee.

Bentsen had sent out invitations to 200 political action committees and lobbyists telling them he was seeking advice and assistance for his campaign.

The council held only one meeting before Bentsen closed it down and returned the money he had collected.

In his last election, 1982, Bentsen raised and spent close to $5 million, which made him the fifth biggest-spending Senate candidate that year, FEC files show.

On his personal financial statement this year, which is separate from the campaign fund, Bentsen listed income of between $296,733 and $418,225 and up. The forms are designed in a manner that does not require disclosure of a specific amount.

His assets included a Warwick condominium and furnishings in Houston, a qualified blind trust from which he received at least $100,001 in interest last year, a note to Brazoria Lake inc. of Brazoria County, Texas, and Bellfield Farm on the Shenandoah River in Clarke County, Va.

Last year, Bentsen's wife earned $61,000 in directors fees for service on the boards of 13 Houston-based mutual funds, according to the senator's personal financial disclosure statement.

Bentsen was born into wealth. His father, ''Big Lloyd'' Bentsen, migrated to the Rio Grande Valley after World War I and became a rich rancher by buying land and reselling it.

Bentsen left the House without seeking reelection in 1954, and became president of Lincoln Consolidated, a financial holding company.