The degree to which the presidency is losing control of foreign policy to Congress is dramatized by two private meetings in which the Carter administration felt compelled to bring members of Congress into face-to-face dealings with foreign leaders in trying to repeal the Turkish arms embargo.

Meeting No. 1: Late last month, Secretary of State Cyrus Vance invited five congressmen fighting to repeal the arms embargo into his office to plot strategy with none other than the Turkish ambassador.

Meeting No. 2: On Sunday evening, June 4, congressional leaders of the Greek lobby went to Manhattan for 2 1/2-hour negotiating session with Turkish Prime Minister Bulent Ecevit in a United Nations Plaza hotel suite. Vance himself helped arrange the meeting, using U.N. Ambassador Andrew Young as intermediary.

Those two meetings reflect President Carter's frustration in conducting foreign policy after the drain of executive power following Vietnam and Watergate. Indeed, he faces the possibility of a humiliating failure to life the three-year-old embargo, with serious consequences both for the Western alliance and his own worldwide prestige.

The strategy session in Vance's office infuriated the Greek lobby, whose own activities had prompted it. "Outrageous!" one eminent pro-Greek leader told us. "That's the first time I know of that members of Congress have met with the ambassador of a foreign country in the secretary of state's office to plot Capitol Hill strategy."

The Sunday-evening between Ecevit and the three principal leaders of the Greek lobby - Rep. John Brademas (D-Ind.) Rep. Ben Rosenthal (D-N.Y.) and Sen. Paul Sarbanes (D-Md.) solved nothing. The congressmen could not negotiate Turkish troop and territorial withdrawals from Cyprus.

Ecevit could not persuade them that continuing the arms embargo would prove fatal to a strong North Atlantic Treaty alliance.

Brademas and his colleagues returned to Washington more determined than ever to defeat the president's efforts to lift the arms embargo. Majority Whip Brademas, No. 3 Democrat in the House, is now putting finishing touches on a bill of particulars that charges the Carter administration with systematic and intentional efforts to undermine the congressionally imposed embargo.

In a private meeting with Vance, Brademas has upbraided the secretary of state for an interview given the Turkish press by U.S. Ambassador Ronald I. Spiers. That interview, Brademas charged, virtually promised that Turkey would soon be rid of the embargo. He will also argue in the House debate that the administration found loopholes to get NATO arms to Turkey.

Accordingly, despite their decision bringing Congress directly into the negotiations with Turkey, the president and secretary of state have failed to move Brademas and other hard-core members of the Greek lobby. On this fight, in contrast to battling for the Panama treaties and the Saudi aircraft deal, Carter is opposed by one of the shrewdest of his own legislative leaders in the House.

Carter's demand for repeal of the embargo, an about-face from campaign statements supporting it, was caused by his perception of rising Soviet military power in Central Europe. But that argument carries surprisingly little weight with Brademas and the Greek lobby.

Dismissing the Soviet angle as unimpressive, Brademas and his colleagues will wield the president's own human-rights campaign as a clincher against repeal of the embargo. Their argument: Carter has turned a blind eye to alleged human-rights violations by Turkey on the island of Cyprus.

The president may be in a larger and tougher fight than he bargains for. Warnings to the White House from friendly congressmen strongly urge a national campaign, in the two weeks before the House vote, as intense as the White House push for the Canal treaties and the Saudi arms deal. Illinois Republican Rep. Paul Findley has passed word to the president that a fireside chat, putting Carter's prestige on the line, may be the only way to defeat Brademas.

But the administration's campaign seems half-hearted by comparison with the earlier battles. On May 25, key congressmen backing the Carter policy went to a White House strategy session they expected to be attended by Carter, Press Secretary Jody Powell and chief aide Hamilton Jordan. None of them showed up, leaving chief congressional lobbyist Frank Moore to hold the fort.

A loss for the president would be a political humiliation. More to the point, it would show that, even when key members of Congress - both for and against - were deliberately brought into his negotiations with another country, he could not assert his will as president.