President Clinton fired a back-to-school challenge to Republicans yesterday, urging that Congress pass a $25 billion plan to rebuild the nation's schools, while he helped Virginia Democrats launch their fall campaign.

At a 75-year-old public elementary school in Norfolk, Clinton pressed the GOP-controlled Congress to approve his construction financing program to aid communities nationwide in modernizing 6,000 schools. Then he picnicked with Sen. Charles S. Robb (D-Va.), sponsor of the school construction bill, Rep. Robert C. "Bobby" Scott (D-Va.) and Democratic legislative candidates.

Clinton's Labor Day swing opened an autumn offensive aimed at congressional Republicans returning to work today as well as at Virginia's Nov. 2 election. With all 140 seats at stake, Republicans are threatening to take over the General Assembly for the first time.

While Virginia Democrats have spent the summer pressing Republicans on transportation issues, both state parties expect the elections will reflect the national debate on education as the fall campaign develops.

Labor Day saw Democrats and Republicans addressing education issues at picnics and national news conferences.

"I'm not against cutting taxes, but I think we ought to take care of the big, long-term challenges of America," Clinton told a crowd of 1,000 at Coleman Place Elementary School in Norfolk, casting the budget showdown with congressional Republicans seeking a $792 billion tax cut as a once-in-a-generation fight over what to do with the nation's surplus.

The GOP plan could halve some federal education aid programs, said Clinton, who has promised to veto the tax cut. "It means larger classes, fewer students, more trailers and more leaky roofs," said the president, who donned a hard hat to help build a school computer table by driving a dozen screws.

One-third of U.S. public schools need extensive repair or replacement, according to the General Accounting Office, which estimates a $112 billion repair bill.

Virginia Democrats expect to outline an education agenda this week that includes school construction funding, higher teacher pay, 2,000 new teachers to cut class sizes from elementary through high schools, more school police and counselors and a "zero tolerance" policy on guns in schools.

At the school and later at Scott's 23rd annual picnic, on a gray, windswept afternoon in nearby Newport News, Clinton stood with Robb and Scott and a dozen state legislators, showcasing "broad and deep support" for the Democrats' education agenda.

Clinton delivered a hearty endorsement of Robb, boosting his 2000 reelection effort against former governor George Allen (R). He praised Robb, saying, "He has always stood up for you. . . . Next time the election rolls around, I want you to stand up for him."

Clinton also delivered a thank-you to Scott, one of his most articulate supporters on the House Judiciary Committee during last year's impeachment debate.

State Sen. J. Randy Forbes (Chesapeake), chairman of the Republican Party in Virginia, jabbed at Clinton and Robb, saying that "with our children going back to school, it's an appropriate time for both parties to roll out the role models."

"Certainly I think Mr. Clinton and Mr. Robb are their [Democrats'] role models, and we're happy to compare them with ours, Governor Jim Gilmore and former governor George Allen, any day," Forbes said.

Republicans have endorsed Gilmore's long-standing pledges to hire 4,000 more teachers, improve school safety, expand early reading and alternative teacher licensing and spend $510 million to raise education standards.

Like the Democrats, Republicans were busy at holiday picnics, where they turned out with their families and campaign staffs to eat barbecue and shake hands. In Mount Vernon, candidates mingled with crowds outside Good Shepherd Catholic Church during its annual International Festival.

Campaign stickers were plastered on shirt fronts, and candidates--their faces glowing with perspiration--shook hands with hundreds of supporters.

"I've moved some of your signs so they're more visible," campaign volunteer Rose Marie Cauthers told Republican legislative candidate Scott T. Klein.

"Well, thank you!" Klein said, reaching out his hand. A 34-year-old native of the area, where he was student body president of his high school and an Eagle scout, Klein is running against Kristen J. Amundson (D) for the 44th District delegate's seat being vacated by Del. Linda T. "Toddy" Puller (D-Fairfax).

Shaking hands with a stream of well-wishers nearby was Republican Daniel Rinzel, who is running against Puller for the 36th District Senate seat in Fairfax being vacated by retiring Sen. Joseph V. Gartlan Jr. (D).

"The Democrats push for new [education] programs with new names attached, as opposed to getting end results, to setting high standards and making sure that children are prepared for the real world," Klein said.

Like Klein, Rinzel said education was a chief focus of his campaign. "Where have Democrats in the state put education?" he asked. "All they've been talking about is traffic, traffic, traffic. The silence on education has been noticeable. For me, it's the most important thing."

Hsu reported from Norfolk and Smith from Mount Vernon.

CAPTION: President Clinton walks between class trailers with student Le'Shia Jamison during a visit to Norfolk to promote school rebuilding programs.