Lebanon's Christian militia forces elected a new commander today in a move seen as strengthening President Amin Gemayel's hold on the Maronite Christian community.

At the same time, leftist groups announced the creation of a new national coalition under the leadership of Druze leader Walid Jumblatt.

Both developments underscored Lebanon's shift away from U.S. and Israeli alignments and bolstered Syria's position as power broker in the Lebanese political crisis.

The Lebanese Forces, which includes several Christian militias, selected Gemayel's nephew, Fuad Abu Nader, as its new commander.

Nader, a 28-year-old physician, replaced Fadi Frem, who has publicly criticized Gemayel's swing to Syria that followed the February collapse of the U.S. peace-keeping effort in Lebanon. Frem has also denounced Gemayel's abrogation in March of Lebanon's troop withdrawal pact with Israel, an agreement brokered by the United States.

Frem, 31, is Gemayel's brother-in-law, but he had assumed command of the Christian militias under Gemayel's more radical brother, Bashir, who as president-elect of Lebanon was assassinated in September 1982. Frem's term expired last weekend.

Gemayel still confronts substantial opposition in Christian ranks over his split with Israel and concerns that he may be selling out Christian interests to Syrian-backed Moslem parties.

Tensions surrounding the elections were reportedly responsible for the recent mobilization in East Beirut of Lebanese Forces units, and possibly for clashes between Christian fighters.

Meanwhile, Druze leader Jumblatt announced the formation of a new leftist coalition that was shaped in coordination with Syria.

In addition to Jumblatt's Progressive Socialist Party, the new National Democratic Front, links the pro-Syrian Arab Baath Party, the pro-Syrian Arab Democratic Front, and the National Syrian Social Party with the Soviet-oriented Lebanese Communist Party.

The movement thus appears to be both a potential vehicle for expanded communist influence in Lebanon as well as a platform for Syria to promote its policies in Lebanon.

Notably absent from the coalition is the main-line Moslem Shiite movement, Amal, which has in the past stayed out of such groupings to assert its independence.