Virginia's gubernatorial candidates, in the last scheduled televised debate of the campaign, finally found some sharp differences in their campaigns, disagreeing on how faithfully they support the Reagan administration and bitterly denouncing each others' negative advertising campaigns.

Democrat Charles S. Robb, who in the past has chosen to tone down his differences with President Reagan, tonight emphasized his disagreement with the Republican president's proposed social security cuts and tuition tax credits for private-school children. He also criticized the president for not reinstituting the draft.

"Virginians should have a right to expect more than political loyalty to a popular president," said Robb.

Coleman, whose major campaign theme has been his faithful allegiance to the GOP White House, contended a vote for Robb would be "a vote against the common-sense government and new federalism" of what he called "the Reagan era."

Coleman began the debate by announcing that "the White House has asked me" to help establish guidelines for urban "enterprise zones" to encourage private economic devleopment and that the city of Norfolk would be the site of a pilot project.

The two candidates saved their sharpest exchange for their own advertising campaigns.

Coleman declared he was incensed by a Robb radio advertisement portraying the Republican as soft on drugs.

Robb mocked Coleman for protesting about negative advertising, saying, "Marshall, your nose should have grown with that particular recitation," a reference to the story of Pinocchio, the boy whose nose grew longer when he told lies. Robb charged it was Coleman who first turned to negative campaigning, calling him "the man who has made it an art form."

As he has done throughout the campaign, Coleman sought to define nearly a dozen differences in issues and philosophy with Robb. He sought to tar the conservative Democrat not only with the Great Society programs of Robb's father-in-law, the late president Lyndon B. Johnson, but also with the "big-spending" schemes of dead liberal Democrats such as president John F. Kennedy and vice president Hubert Humphrey. The GOP candidate charged that Robb's proposals to cope with youth unemployment and problems with the state's public schools were modeled after "Great Society" solutions.

Robb, on the other hand, accused Coleman of vainly searching for "emotional litmus issues," and, turning to his opponent, he chided: "I've got to tell you I don't think the people of Virginia are buying it."

The real issue, said Robb, is leadership, and turning Coleman's campaign motto to his advantage, he added, "It's not enough just to keep a good thing going -- we're going to have to do much better."

The two candidates did not disagree on everything. On the first question asked at the debate, sponsored by the League of Women Voters, both agreed that Virginia's conflict-of-interest laws need to be more sharply defined, although neither proposed specific changes.

Tonight's debate, which reflected a month of increasingly acrimonious and personal attacks by the candidates and their surrogates, was the third televised confrontation in the campaign. Coleman appeared more controlled and stuck closer to the points of differences he set out at the beginning of the program, while Robb appeared to ad lib frequently and, during one answer, even forgot the point he was trying to make and stood silently for an embarrassing moment before recovering.

Both sides claimed victory. "We stuck to our game plan -- Marshall did exactly what he set out to do," said Coleman press aide David Blee. Robb spokesman George Stoddart said: "Chuck nailed him."

The debate also introduced a new element -- racially tinged issues where Coleman again sought to draw sharp contrasts between his brand of conservatism and Robb's.

Coleman cited Robb's support for the D.C. Voting Rights Amendment and voter registration by mail -- both of which are supported by the state's black leadership while opposed by white conservatives -- and attacked Robb for endorsing what Coleman called a racial "quota system" in awarding state contracts.

The GOP candidate also defended a recent newspaper advertisement, placed by local Republicans in the Southside Virginia city of Danville, part of the state's conservative Bible Belt, that accused Robb of "a liberal sell-out" for his stand on these issues.

Robb said the advertisement had "precisely that racial appeal," and called it despicable. He added that it would be "a terrible mistake . . . to set the clock back" by emphasizing racially tinged issues in the final 2 1/2 weeks of the campaign.