One of Washington's premier power brokers, transplanted Texan Robert S. Strauss, yesterday gave a Senate subcommittee an object lesson in how personal power and access to the president often carries more weight than a position on an organizational chart.

Strauss related yesterday to a rapt audience at a hearing on Reagan administration plans to reorganize the government's trade apparatus that, soon after his appointment as special trade representative in the Carter administration, he learned he was not included in the official delegation to the 1977 economic summit in London despite the strong role trade would play in the discussions among the Western allies.

A White House aide said he wasn't needed at the summit. "Send a memo," Strauss said he was told.

"I'm going to give you a resignation, not a memorandum, and I want your answer in the next few minutes," Strauss said he replied to the White House aide, who he declined to identify.

"The president came back with his answer, and I went to London," Strauss concluded.

The moral of that story is that any new government appointee must set himself up as a strong person early, he said. "If I hadn't, there never would have been a Tokyo Round of negotiations," completed in 1979, which began to attack nontariff barriers to trade.

In the course of his two hours of testimony before the Senate Committee on Governmental Affairs, Strauss, now in private law practice, managed to appear to agree with all the senators without firmly committing himself to any position in the current battle over the management of America's trade policy.

He agreed with Sen. John C. Danforth (R-Mo.), who opposes the trade reorganization measures for their possible dilution of substantive efforts to improve America's trade position, that "there is no magic wand in creating a new department." But he also agreed with Chairman William V. Roth (R-Del.) that there is a void in the development of America's trade policy.

Strauss attacked the Reagan administration for failing to supply "strong, aggressive commitment from the top" on trade isues. "This administration's rhetoric is very good, but it's performance leaves much to be desired," he said.

But he asked for a chance to testify later in the hearings, which have assumed added importance since the Reagan administration announced Monday that it now favors establishing a special Department of Trade, formed from the White House's small Office of the U.S. Trade Representative and parts of the Commerce Department. The hearings originally had been called on Roth's bill to form a "lean, mean" Department of Trade.