Republican nominee-to-be George Bush picked up what could prove to be an unwelcome piece of baggage on the campaign trail - President Reagan's veto of a massive trade bill. Democratic presidential contenders Jesse Jackson and Michael Dukakis, meanwhile, were facing off Wednesday night in their first one-on-one debate in a month.

Bush cruised to an easy victory in Idaho's presidential primary on Tuesday. He was picking up 15 of the 18 national convention delegates, as Dukakis won a non-binding Democratic "beauty contest" with no delegates at stake.With 99 percent of the precincts reporting, Dukakis was taking 73 percent to Jackson's 16 percent in Idaho, while Bush was gaining 82 percent to 9 percent for Pat Robertson, who suspended his presidential bid earlier this month.

Robertson was winning one delegate, with two going to the uncommitted column. Bush already has more delegates than he needs for the GOP nomination.

The vice president was leaving Washington today to spend a week at his summer home in Kennebunkport, Maine. He was expected to keep a low profile to avoid upstaging Reagan during the superpower summit.

As Reagan was leaving Wednesday morning for Moscow, Bush was heading for West Point, N.Y., to give a commencement address at the military academy.

Wednesday night's Jackson-Dukakis debate in San Francisco comes as the two campaign for California's June 7 primary. Candidates' debates were almost painfully commonplace earlier in the campaign, as the then-crowded Democratic field met frequently for talkfests.

But the last time Jackson and Dukakis met for a debate was in April, prior to the Pennsylvania primary. Back then, the two were exchanging mainly pleasantries. But Jackson has been more critical of Dukakis in recent weeks as the delegate gap between the two widened.

Dukakis is now within striking distance of the 2,081 delegates needed to nominate. He has 1,669 to Jackson's 983, according to the latest Associated Press count.

The Democratic front-runner was in Washington Wednesday, where he was picking up an assortment of congressional endorsements. He also had a meeting set with former rival Albert Gore Jr., the senator from Tennessee, who suspended his Democratic presidential campaign a month ago.

On Tuesday, Dukakis denounced Reagan's veto of the trade measure, which would require advance notice of plant closings.

"Any administration that is willing to give Gen. (Manuel Antonio) Noriega 120 days notice and a plea bargain ought to be willing to give the American worker 60 days notice before they are thrown out on the street," the Massachusetts governor said, referring to talks aimed at easing the Panamanian military leader from power. Noriega is under indictment on drug charges.

Dukakis made it clear the plant-closing issue would be used as ammunition in his expected campaign against the vice president. Reagan and Bush, he said, "don't understand that for working families . . . there is nothing more painful than losing a job."