TEMPE - A defiant Pat Buchanan bragged in a freewheeling Republican debate yesterday that the Japanese government was nervous he would win the presidency and demand fairer trade rules. He vowed, if elected, to ''unilaterally'' end Japan's trade surplus if necessary.

Before a noisy auditorium crowd at Arizona State University, Buchanan also described his plan to build a security fence on the U.S-Mexico border to ''stop the illegal immigration along the Southern border of this country cold.''

Former Gov. Lamar Alexander, combining a critique of Buchanan's trade and immigration views, suggested his rival was a defeatist who wanted to ''build two walls around America.''

Buchanan, the New Hampshire primary winner, was the center of attention - and attacks - throughout a crackling 90-minute televised event five days before Arizona's hotly contested GOP presidential primary. Buchanan often shouted as he fought back, and had to be stopped several times by the moderator for running over his allotted time.

Senate Majority Leader Bob Dole took a few barbs early for being absent, but then was all but forgotten as Buchanan, former Tennessee Gov. Lamar Alexander and publishing heir Steve Forbes clashed pointedly over trade, tax reform, immigration, and, more broadly, the future direction of the Republican Party. California Rep. Robert Dornan also participated, often acting more like a referee than a candidate himself. Not that there weren't areas of agreement.

All four pledged to shift control of welfare to the states, and said local control over national parks and other lands now controlled by Washington was a good idea, too. And in a state where 13 percent of the population is 65 or older, all four said current Social Security recipients needed to be protected while reforms to the retirement system are discussed.

But disagreements dominated.

''Pat, I am not going to let you hijack our party,'' Alexander said at the outset. Throughout the evening, he said ''Buchananism'' would hurt the economy by ''building a wall around the country'' with protectionist trade views.