With outspoken criticism of the recent Vietnamese-led invasion of Cambodia, Romania once again has challenged the Soviet Union on a major point of international policy -- a stance of defiance that is becoming increasingly frequent and open.

In the past few months, Romania has given a warm welcome to China's leader Hua Kuo-feng, has refused Kremlin demands for increased defense spending and has approached the European Common Market for talks on a commercial pact. All of this is calculated to anger the Soviets.

With the new rift over events in Indochina, the gap between Moscow and Bucharest seems wider -- in the view of specialists here -- than at any time in the past decade.

While other Soviet allies last week speedily endorsed Vietnam's ouster of the Khmer Rouge government in Phnom Penh, a Romanian statement declared that the offensive "was a heavy blow for the prestige of socialism."

"No reasons and arguments whatsoever," the Romanians asserted, "can justify intervention and interference in the affairs of another state, whatever their form, especially when two socialist countries are involved."

The Romanian language, analysts recalled, was similar to that used at the time of the Soviet-led invasion of Czechoslovakia in 1968 when Bucharest was rife with fears that it too might be attacked.

Obviously there is no such danger now. Yet Romania's alarm about the Soviet-backed Vietnamese action half a world away apparently reflects fears in Bucharest about a pattern of stronger Communist governments replacing weaker ones of which they disapprove.

For years, Romania's leader Nicolae Ceausescu has carefully crafted his country's independent foreign policy by taking, as one analyst put it, "two steps away from Moscow and one back." Lately, all the steps have been in one direction -- away.

While the Kremlin no doubt is upset about the Romanian moves, U.S. officials have spotted no concrete Soviet moves to punish Ceausescu. "The Soviets have been surprisingly restrained in their response given the provocation," one senior official remarked yesterday.

Ceausescu has held no meetings with top Soviet officials since a Warsaw Pact summit in November. Yesterday, however, it was reported from Moscow that Romanian Foreign Minister Stefan Andrei will visit the Soviet Union later this month.

That session could provide the forum for a stern Soviet rebuke to the Romanians.

Moscow has learned over the years how to reconcile its concern over Ceausescu's outspokenness with his need to win favor among the Romanian people. And defying Moscow is easily Ceausescu's most popular position at home.

But by piling up opposition to Soviet moves in such public ways, Ceausescu, in the view here, could be testing the limits of Kremlin tolerance.

Why Ceausescu has chosen to be so bold now is a bit of a puzzle.

The best explanation is a combination of domestic and international factors. At home, Romanian independence moves tend to coincide with periods of national celebration, such as a recent one noting the creation of the Romanian nation-state.

Abroad, Bucharest plainly feels that with statements on Cambodia it will win valuable points among the nonaligned countries that Romania seeks to impress.

For the Kremlin, the Romanian challenge is a persistent irritation, one which it thus far has been unable to repress.