Virginia's overwhelmingly Democratic General Assembly quickly adjourned its 1979 session today after its leaders conceded Republican Gov. John N. Dalton had thwarted their plans to override any vetoed legislation.

In a surprise move, Dalton blocked the Democrats from even attempting an override by sending the legislature amendments -- instead of a veto -- on a highly partisan bill aimed at Dalton's fellow Republican, Attorney General J. Marshall Coleman.

The young attorney general is considered the GOP's likely nominee to succeed Dalton in 1981, and he has many political enemies in the Assembly.

Democrats had passed legislation stripping Coleman of his power to assign highway condemnation cases to private lawyers and had assumed that Dalton would promptly veto the measure.

But Dalton upstaged the Democrats, sending the Assembly amendments that gutted the bill on Coleman. The state Senate today rejected the amendments, but their action -- required by the state constitution had the effect of giving Dalton additional time to consider whether or not to veto the bill. Since the Assembly adjourned for the year today, Dalton was left free to veto it without the threat of an override.

"This is a very clever way of removing from us the opportunity to override a veto," said Senate Majority Leader Adelard L. Brault (D-Fairfax), who has long wanted the legislation to flex its rarely used veto power. "I am disappointed that he sent us this amendment this way."

In contrast to Brault, Republicans were smugly pleased by Dalton's maneuver. "I think it's a triumph for the governor," Del. Vincent F. Callahan (R-Fairfax), chairman of the House-Senate GOP caucus, said in an interview. "I think he's outbluffed them."

Sen. Hunter B. Andrews (D-Hampton), who sponsored the Coleman bill, airily dismissed suggestions that Dalton's action resulted from a high-level strategy session within the GOP administration. "It didn't take an Einstein to figure out how to do this," he said.

The Virginia legislature has been extremely reluctant to override a gubernatorial veto and some Democrats were hoping that this year, for the first time in recent decades, the Assembly would over-turn a veto.

Today's a one-day legislative session was planned to give them that opportunity and to correct any minor problems in legislation approved in a 45-day session that ended seven says earlier. Under Virginia law, Dalton has to decide within seven days whether to sign or veto any measures pressented to him while the Assembly is in session.

If the Assembly has adjourned, he has 30 days in which he can decide whether or not to approve legislation. By forcing the Assembly to vote on amendments today, Dalton effectively added 30 days to the time in which he can consider controversial measures and avoided the prospect of an override.

One possible veto confrontation evaporated when the Senate Democrats decided today to let stand Dalton's veto of a bill that would have permitted optometrists to use certain drugs when performing eye examinations. Use of the drugs in question is now limited to ophthalmologists, who are medical doctors.

The House overrode Dalton's veto more than a week ago, but it was obvious to Democratic leaders that there were not enough votes in the Senate to get the necessary two-thirds majority.

Largely because of constitutional limits on its power, the Assembly has overriden only two votes by Virginia governors since 1927. The legislators this year approved a proposed constitutional amendment that would allow the Assembly to reconvene to override vetoes in the future. The amendment will become effective in 1981 if it is approved by the Assembly again next year and then ratified by voters.

Coleman, the state's first Republican attorney general, angered Democrats and some highway officials last year by replacing longtime Democratic condemnation lawyers with Republicans. The Andrews bill would have stripped him of much of this patronage power by assigning the work to city and county attorneys in localities that have full-time legal staffs.

Dalton's proposed amendment would have given the attorney general the right to decide whether local government or private lawyers would do the work, effectively nullifying the change proposed by Andrews.

Brault said Dalton told him of his plans yesterday, but left him with the impression that his amendments would give city and county attorneys, not the attorney general, the right to decide who does the condemnation work. "I don't want to say I was led down the primrose path by the governor," Brault said, implying that he was.

With the highway lawyer bill disposed of, the Senate and House of Delegates plowed through mostly minor amendments to 21 other bills returned by the governor for changes and adjourned at 1:11 p.m., formally ending a session that began on Jan. 10.

Occasional lulls in the final work of the Assembly were filled today with speeches praising retiring members and informal conversations about coming political campaigns. All 140 Assembly seats are up for election this year.

For Sen. Omer L. Hirst (D-Fairfax), the senior legislator from Northern Virginia, this was the last day of a 22-year Assembly career. When Lt. Gov. Charles S. Robb announced the final Senate adjournment, Hirst quietly picked up the green blotter that covered his desk and walked from the senate chamber for the last time with the souvenir folded under his arm.