When House Speaker Jim Wright (D-Tex.) embarked last summer on a high-wire mission in Central American peacemaking, his closest colleagues warned him off. One even shouted at him that he was walking into a White House trap. Yet the adviser he most trusted insisted he go forward, and go he did.

When Wright raised the stakes higher, meeting this month with Nicaraguan President Daniel Ortega, provoking ear-splitting outrage from the administration, the speaker again took his cues from that one adviser.

Asked last week whom the maverick Texan sought counsel from in this most sensitive of initiatives, helping him weigh the stakes for foreign policy and for the credibility of his office, a senior aide answered matter-of-factly:

"Himself."

During almost a year as speaker, Wright has emerged as a political lone ranger, leading Democrats into high-risk territory often before they are ready to go. His guide at each juncture has been this inner circle of one.

Wright's approach marks a dramatic shift in the running of the House and in the role of the House speaker as Washington's No. 1 Democrat. As of last week, he appeared to be succeeding beyond all expectations, and praise -- at least among fellow Democrats -- for his boldness was almost loud enough to muffle a steady drumbeat of concern over its risks.

Last Tuesday, with House Democrats solidly behind Wright's Central American peacemaking foray, and both factions in Nicaragua responding positively, Secretary of State George P. Shultz found himself making peace with the speaker.

On the fiscal front, President Reagan, who once said taxes would increase "over my dead body," joined Wright and other congressional leaders from both parties on Friday at the White House to call for $23 billion in tax increases by 1989 in the name of cutting the deficit and reassuring world financial markets.

But Wright the man, more than the results he helped set in motion, became the topic of interest here last week. One House Democrat, who insisted on anonymity, said that Washington is still accustomed to the more languid style of former speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.), who "let the system bring issues to the surface and moved the pieces around once they came to the top."

In some ways, he said, Jim Wright is just what the party ordered: a bold challenger of Reagan who articulates a Democratic agenda as the party girds for an opportunity to reassert itself in the 1988 elections.

"But Jim is more aggressive, even precipitous, compared to Tip. And that carries risks," the Democrat continued. "Right now, the support among Democrats is strong. But there's no question it creates a nervousness. We're never sure where he's going to lead."

Republicans were far less charitable. "He's the most arrogant abuser of power I've ever seen," said Minority Whip Trent Lott (R-Miss.). "With Tip, there was warmth and affection," said Rep. Dick Cheney (Wyo.), chairman of the Republican Policy Committee. "There are no such feelings for Jim Wright."

"Jim Wright is a fellow who's had some bobbles in the early days of his speakership," explained ubiquitous Texan Robert S. Strauss, a friend who stepped in last week to orchestrate the administration's truce with Wright. "But he has political courage. I'd rather have a fellow make a few mistakes and have the courage to provide leadership than have one too timid to stick his neck out."

"If you think about the turtle," Wright said, "until he sticks his neck out of that shell, he makes no progress."

Even in appearance, Wright, at 64, is a man apart. Silvery-haired and dapper, he has a thicket of eyebrows that look something like a handlebar mustache that took flight and landed just above his aviator glasses. His soft, courtly, Texan voice conveys serenity even when he is angry. His folksy brand of humility is most in evidence when he seems most sure of himself.

"It may be that I'm unwisely bold," Wright said in exaggerated self-effacement of his highly publicized role in urging Nicaraguan factions to accept the mediation of Cardinal Miguel Obando y Bravo. "I hope that isn't the case."

Wright's involvement in that effort was very much in character, taking him much further into the machinations of policy than is customary for a House speaker. As Wright and Rep. David E. Bonior (D-Mich.) tell it, Wright even helped set in motion Reagan's invitation to him last August to become a partner in a bipartisan bid for a peace agreement in Nicaragua.

For months, Wright sought meetings with White House officials, warning that Congress was likely to reject further aid to the Nicaraguan contras, particularly as the cloud of the Iran-contra scandal darkened, according to Bonior, a leading opponent of contra aid. The speaker said his message was that the White House should seek an alternate approach to peace in the region.

Wright, who is fluent in Spanish, meanwhile was pumping contacts of his own in Central America, born of longstanding interest and travel in the region as well as a number of diplomatic assignments there since the late 1970s.

He said he talked with Costa Rican President Oscar Arias and Salvadoran President Jose Napoleon Duarte, and knew before joining hands with Reagan last August that the five Central American leaders were likely to put forth a peace proposal of their own within days of the Wright-Reagan accord.

When they did, Wright quickly embraced it, while the administration hung back, terming the plan too easy on the Marxist Sandinistas. Despite the mixed signals here, the peace process was alive. House Democrats, who in August believed Wright was joining an empty peacemaking gesture that would set up emboldened White House calls for military aid to the contras, gave him credit when the administration recently bowed to the political momentum and said it would seek no more such aid this year.

"The White House thought they had a patsy on their hands and they could walk him into a trap," said Rep. Beryl Anthony Jr. (D-Ark.), chairman of the Democratic Congressional Campaign Committee. "He showed them what they really had on their hands was a wily politician."

In ensuing weeks, Wright met with contra emissaries in Fort Worth and Washington and kept up contacts with both factions. He said the Sandinistas twice asked him to mediate the negotiations, but he said he pushed for Obando instead. When the Sandinistas balked, he said, "I became a little impatient," and pushed harder.

He said he became convinced that the administration approach of denying an audience to Ortega would never lead to a cease-fire. "How do you negotiate a cease-fire," Wright said, "when the Sandinistas would talk to nobody but Reagan, and Reagan and the White House were saying there could be no talks except between the Sandinistas directly and the contras? . . . . You had to have an intermediary."

Typical of his approach to legislation in the House, Wright said he even advised Ortega on what to include and exclude from his 11-point peace proposal. Specifically, Wright said, he told the Nicaraguan leader to drop any hostile references to the United States and to vow to restore political liberties, inviting the intermediary to verify compliance with this effort.

"After all, you're trying to make peace," Wright said he told Ortega.

This was the detail man in Wright, an integral piece of his personality, according to all who have worked with him. Despite his enormous workload, he makes time to summon reporters and to critique their articles about him, down to their word choice and placement of his quotes.

Unlike O'Neill, who deferred ceremoniously to committee chairmen, Wright publicly suggests how they should write legislation -- from taxes to budgets to insurance of the troubled savings and loan industry. It is an approach that has infuriated some senior chairmen. Others, however, see in it a prod to action.

"As a Ways and Means Committee member, I see that as an encroachment," Rep. Robert T. Matsui (D-Calif.) said of Wright's call last year for higher income-tax rates for the wealthy before discussing the idea with the tax-writing panel. "But stepping back, it probably served to get the ball rolling. When we did produce our bill {minus the higher rates}, he was very supportive of it."

Nowhere did the elements of Wright's leadership converge with more fireworks than in the Herculean fight last month over a $12.8 billion tax increase.

As in his Central American initiative, Wright was urged by many in both parties to hold back. A bipartisan budget summit was afoot in the wake of the Oct. 19 stock market plunge, and to go forward with a significant, Democratic-backed tax increase could infect the spirit of common purpose with partisanship, the doubters warned.

Wright's view, however, was that such a bill would pressure the summit to produce a significant tax increase; without that, he said from the start of his speakership, the deficit would never be tackled. If the summit collapsed, he said, this would be a fallback.

"Sometimes a spokesman for effective political change must await the development of public support," a philosophical Wright said in an interview last week. "But if he sits back and passively waits for a public uprising, the problem will be so severe it defies any solution."

Wright assembled a package intended to win the support of his party's formidable liberal wing as well as conservatives. The liberal sweetener: an amendment requiring a $6 billion welfare-overhaul plan. The conservative one: a $3 billion increase in defense appropriations.

Instead, the package angered both sides. Conservatives, including a fellow Texan, Rep. Charles W. Stenholm (D), felt that Wright had gone back on an agreement not to include a welfare-spending initiative in what was billed as a deficit-reduction bill. And a number of liberals felt that he gave only half-hearted support to the welfare package, leading to its defeat.

When the package failed by 14 votes on the House floor last month, Wright turned aside a request by House Minority Leader Robert H. Michel (R-Ill.) to leave the matter to the budget summit. The welfare proposal was excised, and the thinner package returned to the floor for a second try.

This time, a significant number of liberals bolted. With the clock on the vote board showing no time left, the package appeared defeated 206 to 205. Wright then held the vote open -- unusual but not unheard of -- and 10 minutes later one of his fellow Texas Democrats, Rep. Jim Chapman, surfaced to change his vote to "yes."

Three weeks later, Republicans remained incensed. Minority Whip Lott said House GOP conferees at the deficit summit were loath to consider Democratic options because of lingering ill will. "There is no reason for Jim Wright to expect significant Republican help," said Rep. Willis D. Gradison (R-Ohio), usually a moderate voice in his party. "There are some things an elephant never forgets."

Wright said he believes Republicans are simply venting frustration over their inability to thwart his programs. "Short of just letting the minority take over and dictate the schedule, I honestly don't know how I could be more considerate," he said.

There are those in the House who believe that events involving the budget battle and Central America have shaken Wright and his leadership style. Some even predict more caution, perhaps a new awareness of the reluctance of some Democrats to climb onto high wires with him.

"Of course you learn from each experience," Wright said last week, sounding almost chastened. Asked what he learned, the lone ranger in Wright responded:

"I learned that when you step out front, you need to look around behind you."