Democratic presidential front-runner Michael Dukakis defended his spending priorities and took a mild poke at rival Jesse Jackson's five-year budget blueprint, saying there's "no way" specific spending can be figured so far in advance. And Idaho was holding its primary today, but it was a fairly low-key affair.

There were 18 Republican delegates up for grabs in Idaho, but Vice President George Bush's mathematical lock on the GOP nomination has taken the suspense out of the contest. There were no Democratic delegates at stake; most are to be chosen at a state convention next month.Officials in Idaho said a low turnout was likely.

Dukakis was tending to gubernatorial tasks in Massachusetts Tuesday, and traveling to Virginia for an evening appearance. Bush was in Washington, preparing to see President Reagan off later this week for next week's superpower summit in Moscow.

For Jackson, however, business as usual meant campaigning in California, where primary day is two weeks away.

Jackson released a budget plan on Monday that would raise taxes on the rich, freeze defense spending and boost support for social programs.

The five-year proposal would hike taxes on the wealthy by $244 billion and on corporations by $104 billion. It would cut the defense budget by $164 billion by freezing projected spending hikes, and would increase spending on social programs by $344 billion.

Jackson said his plan would cut the nation's deficit from a projected $134 billion in 1993 to $49 billion.

Dukakis did not take issue with the specifics of Jackson's spending plan, but said his own experience as governor had taught him that budgets cannot be mapped out in such detail years ahead of time.

"There's no way you can prepare a budget today for 1989," said Dukakis, who is struggling to cope with changing revenue projections at home in Massachusetts.