Early in his congressional career, Rep. David E. Bonior (D-Mich.) learned that process can drive substance, and on Tuesday he put the lesson to good use.

Using a procedure devised two years earlier by Bonior, the House Rules Committee structured today's debate on two competing contra aid proposals in a way that seriously undercut Republican efforts to defeat a Democratic proposal to provide the rebels $30 million in purely humanitarian aid.

By placing the Democratic plan as a substitute to the Republican package of $36 million in aid that would favor the Reagan administration's efforts to resume military assistance later this year, the committee guaranteed that the Democratic proposal would be voted on first. That leaves supporters of the president's Nicaragua policy in a quandary: they can oppose the Democratic proposal, but if it passes, there will be no chance to vote on the Republican alternative, leaving them vulnerable to election-year charges that they abandoned the rebels fighting the government of Nicaragua.

The procedural maneuver engineered by Bonior illustrates how the Democrat, who came to Congress 11 years ago as a classic antiwar liberal, has evolved into one of the consummate inside players on Capitol Hill.

And it is a mark of the widespread respect in which he is held in the House that when Republicans howled in protest over Tuesday's action by the Rules Committee, they directed their anger at Speaker Jim Wright (D-Tex.) rather than Bonior, the Democrats' chief deputy whip and task force leader on Nicaragua.

Even as he lambasted Wright for breaking a promise to give House Republicans a vote on their contra aid package, Rep. Mickey Edwards (R-Okla.) praised Bonior as a "gentleman" and credited him with knowing "how to use the power of the majority."

For Bonior, today's vote on the Democratic contra aid plan represents the toughest test he has faced since Wright appointed him last year to the No. 4 leadership post, an appointment designed in part to build a bridge to the party's liberal wing. After several years of spearheading the effort to defeat repeated Reagan administration requests for contra aid, Bonior has had to alter course and convince liberal Democrats and advocacy groups that the best way to prolong the cutoff of military aid to the rebels and to encourage the Central American peace process is to support the leadership's plan for continued humanitarian aid.

"For him to have brokered a package that includes additional aid was a very wrenching experience for him," said Rep. Robert J. Mrazek (D-N.Y.). "He's built a consensus in the most difficult situation imaginable."

Added Rep. Pat Williams (D-Mont.), who may yet vote against the leadership package: "Liberals in the House have a deep, abiding trust in Dave. That trust more than anything has prevented a collapse of votes on our side. It's more based on trust than on policy."

Bonior's success in swaying some liberal members of his party has been a function of his unimpeachable anti-contra aid credentials and of a personal style that makes him one of the most trusted members of the House.

Colleagues and Capitol Hill aides portray Bonior as intensely driven and competitive, but also extraordinarily patient and unassuming. "He's one of the best combinations here of high intelligence and low ego," observed Rep. Barney Frank (D-Mass.).

Bonior's brooding, almost melancholy, demeanor, which is accentuated by a full, black beard flecked with gray, gives him what one Democrat called a "Lincolnesque" quality that fosters sympathy and votes.

"There's kind of a sadness to him," Frank notes. "A seriousness that verges on sadness. It helps him, it makes people feel like 'Let's not make it worse for him.' "

An accomplished athlete, Bonior's choice of solitary but demanding sports reflects his personality. Until two back operations limited his exercise regimen to daily three-mile runs and half-mile swims, Bonior was a triathlete and a marathon runner.

A graduate of the University of Iowa, where he lettered in football, Bonior spent four years on stateside duty with the Air Force during the Vietnam war. During two terms in the Michigan House of Representatives, he earned a reputation as an environmentalist by pushing a state ban on toxic substances such as polychlorinated biphenyls (PCBs).

In his first race for the U.S. House in 1976, Bonior latched onto an election-year gimmick that dovetailed with his politics and has since become his campaign signature. After an ice storm killed hundreds of trees in his district northeast of Detroit, Bonior handed out 50,000 pine seedlings as he went door to door. He has since given out 250,000 "Bonior pines."

During his early years in the House, Bonior specialized in environmental issues and was the earliest congressional champion of Vietnam veterans. He formed a caucus of lawmakers who served during the conflict, cowrote a book detailing the neglect of Vietnam veterans, and helped push through legislation to improve their education, counseling and job benefits by using the rules to go around the Veterans Affairs committees that tended to favor older vets.

"He's a rebel, but he was playing top-flight, inside ball on an issue no insider wanted to play," recalls Steven M. Champlin, a former aide who now works for House Majority Whip Tony Coelho (D-Calif.). "That's the paradox."

The question is whether Bonior can continue to straddle those two roles. On contra aid, one influential Democrat said, "He's been able to bridge that gap . . . . but on other issue he can't. He'll always be influential with the liberals, but can he be a rebel? No, because he's a part of leadership."