Virginia's candidates for the U.S. Senate, speaking jointly for the first time in the campaign, demonstrated last night that their differences are more real tha apparent.

Democrat Andrew P. Miller and Republican Richard S. Obenshain, speaking and answering questions before about 150 leaders of the Virginia Farm Bureau, basically agreed on a wide range of issues from balancing the budget (for) to HEW Secretary Joseph Califano's attacks on smoking (against).

But within that generally similar framework, Miller, the 45-year-old former state attorney general, stressed his own experience and attacked what he called the philosophical "rhetoric" of his opponent.

Obenshain, 42 and former national cochairman of the GOP, blamed mose of the nation's problems on the Democratic Party and the Carter administration and agrued that Miller was tied to both.

Both men were low key and polite in their references to each other before the county president and officers of the state's largest farm organization, but they were hints of a rough edge that could develop as the campaign wears on.

"You two are so jolly, so friendly," said Farm Bureau moderator Albert Moffett during the questioning period.

"Just wait," said Obenshain. And the quips of one man were frequently met with a scowl from the other.

Both candidates, dressed in conservative dark blue suits, gave identical answers to a series of questions - against increased beef imports, against a guaranteed annual income, for a constitutional amendment to require a balanced budget - with one exception.

On allowing states to rescind the Equal Rights Amendment, Obenshain said yes while Miller said such action would be unconstitutional. Miller favors the amendment while Obenshain opposes what he calls the "so-called ERA."

Obenshain also attacked unions as having "gotten out of hand" and called for a national right-to-work law. Miller said unions "are part of American society," but like Obenshain he expressed opposition to a series of national bills backed by labor.

Both men began with statements stressing their farm backgrounds and ties with the Farm Bureau and called for efforts to increase farm exports. Obenshain opposed a proposal for 100 percent of parity for farmers, while Miller backed that proposal, which would return farm purchasing power to its 1910-1914 level.

Miller and Obenshain, both former presidents of their parties' state "young" organizations, favored tax cuts. Obenshain called for an across-the-board 33 percent reduction over three years. Miller said that such a step would create an enormous deficit and called for cuts in government spending.

Obenshain frequently expressed his positions from a highly conservative philosophy. Miller often took a more pragmatic view, attempting to deal with the nuts and bolts of issues.

His approach often carried him over the two-minute limit for answering questions, and at one point Moffett said, "You're harder to handle than Henry Howell."

The audience roared with laughter and Obenshain put both hands over his head and clapped. Miller obviously was not pleased by the comparison with Howell, the populist Democrat who beat him in a bitter gubernatorial primary last year.