MINNEAPOLIS (AP) - One stressing experience and the other his relative youth, former Vice President Walter Mondale and Republican Norm Coleman started up full-time campaigning in earnest yesterday, and their parties squared off before the state Supreme Court over the possibly crucial question of absentee ballots.

The campaign to determine who would succeed the late Sen. Paul Wellstone - a race that could tip the balance of power in the Senate - fired up yesterday, the day after Mondale formally accepted the nomination.

Mondale told reporters he intends to serve a full six-year term, and said his background as senator, vice president and ambassador means he can walk into the Senate and immediately be a leader.

"I don't apologize for my experience. It's an asset," Mondale said at a news conference.

Earlier in the day, Mondale reminded voters of the same thing. "I think I can help on the first day to attack those problems that must be dealt with," Mondale said on WCCO-AM. "Education, environment, the economy is stumbling. We've got some very severe challenges internationally."

Coleman, meanwhile, was spending a second day crisscrossing the state, campaigning yesterday morning at a restaurant in Moorhead. He had halted his campaign last week after Wellstone died in a plane crash.

Three decades after his last Senate run, Mondale also had a full schedule, with several hours blocked off from campaigning to attend a funeral for one of the people who died in the crash with Wellstone.

Also yesterday, the state Supreme Court heard arguments in a Democratic Party lawsuit intended to make sure new ballots are mailed out to give residents a chance to vote for Wellstone's replacement. No decision was issued.

Alan Weinblatt, an attorney for the Democratic-Farmer Labor Party, urged the court's seven justices to rule that existing absentee ballots should be considered "spoiled" if voters who cast them want to change their vote.

But the GOP contended that a massive re-mailing of absentee ballots could actually disenfranchise supporters of other candidates, by leaving them too little time to file their new ballot. Almost 4.5 percent of Minnesotans voting cast absentee ballots in 1998, the last non-presidential election year.

Mondale was seen as Democrats' best shot at keeping Wellstone's seat, and more than 800 party representatives approved his candidacy Wednesday with an exuberant "YEA!" There were no dissenters.

"I think given the circumstances, he's absolutely the strongest candidate we could field," said delegate Buzz Snyder, a postmaster in St. Cloud.