Texas Attorney General Jim Mattox, indicted Tuesday on a charge of commercial bribery, contends that he is the victim of a Mobil Oil vendetta and says he will emerge a "hero" at trial.

But some leaders of Mattox's own Democratic Party worry privately that the grand jury indictment has damaged the credibility of the combative, blunt-spoken, populist ex-congressman, and they say they wonder if the fallout will tarnish the state party's progressive wing.

Mattox, the first sitting attorney general in Texas history to be indicted criminally, is accused of threatening to use his office to ruin the municipal bond business of Fulbright & Jaworski, a 320-member law firm based in Houston.

The alleged threat came as Mattox and senior partners of the firm were feuding over a $1.7 billion mineral rights suit that the state and a south Texas rancher-oilman, Clinton Manges, brought last year against Mobil Oil, a client of Fulbright & Jaworski.

In connection with the suit, Fulbright & Jaworski lawyers sought earlier this year to subpoena Mattox's sister, Janice, seeking testimony about possible improper links between Manges and Mattox in the financing of Mattox's 1982 campaign.

According to a telephone conversation taped by J. Wiley Caldwell, a senior partner at the firm, Mattox allegedly threatened to cut off or delay 17 municipal bond sales involving the firm unless it agreed to back off the subpoena.

Municpal bond work is considered a political plum for law firms. The work is routine and often highly lucrative, since compensation is based on a percentage of the bond deal rather than on hours worked.

Many firms that compete for the no-bid work help bankroll the election campaigns of the public officials who control it--and Fulbright & Jaworski, the nation's 11th-largest firm, is no exception. Its partners raised substantial funds in 1982 for Mattox, among others.

Mattox has said that his taped remarks were never intended as a threat and claims that Fulbright & Jaworski is trying to divert attention from the huge mineral rights suit.

"Mobil has got us where they want us," he said, but "I'll just keep fighting."

The war of words has been hot here all summer. Mattox, 40, who grew up in a blue-collar east Dallas neighborhood and prides himself on being a scrapper, even called for a boycott of Mobil products. The call was resoundingly ignored.

Mattox, winner of six straight elections, always has been been more popular with the voters than with fellow politicians, and it is hard to find much support for him among political figures here. Gov. Mark White, a Democrat who preceded Mattox as attorney general, has called the situation "unfortunate."

Some Democrats fear that the investigation--spearheaded by a local Democratic district attorney--will widen. Manges gave more than $1 million in political contributions last year, most of it to progessive Democrats, and his business affairs keep him in commerce with the state.