As the opening gong for the presidential campaign sounds in Iowa, the United States Congress returns from a holiday break for what could be the last opportunity to wrap up serious legislation before it surrenders to election-year politicking.
First on the agenda will be the airing of lingering concerns about American involvement in Lebanon. Now that US marines are packing up and moving from Beirut to offshore ships, Congress is unlikely to take any action. But many lawmakers have raised questions about the shelling of Moslem and Syrian strongholds around Beirut by American ships, just as the Marine redeployment was announced nearly two weeks ago.
''I believe it's a violation of the agreement (between Congress and the President) under the War Powers Act,'' says Rep. Robert Matsui of California of the US shelling. ''That is, in my view, taking offensive action. I don't believe it's a defense of our marines.''
Representative Matsui, a member of the 14-member Democratic Lebanon task force appointed by House Speaker Thomas P. O'Neill Jr. (D) of Massachusetts, has called for an immediate investigation ''to clear up apparent contradictions'' in administration statements about the justification for the naval shooting.
On the budget front, Congress returns to what appears to be a standoff between Democrats and President Reagan over deficit reduction.
A bipartisan panel has, as of this writing, set no date for a second meeting to discuss ways to reduce the federal red ink, following its first inconclusive session two weeks ago. In classic ''After you . . . '' style, both sides say they're waiting for each other to make a move. An impatient Robert Dole (R) of Kansas, chairman of the Senate Finance Committee, has announced he will proceed this week, with or without the panel, to move on $100 billion in tax options to reduce the deficit.
Meanwhile, what could be the major legislative accomplishment of the year, immigration reform, moves to the fore in the House. The leadership has set March as a target for completing floor action, but the various sides are now girding for the confrontation.
At issue is whether the federal government will impose criminal sanctions against employers who hire illegal aliens and whether employers must begin checking identification documents of their workers.
Both provisions, written into the Senate-passed bill and proposed in the House, face bitter opposition from Hispanic groups. In the last Congress, Rep. Edward R. Roybal, a Hispanic California Democrat, pulled out all of the parliamentary stops to block the reform measure.
As Congress reconvenes today, the Mr. Roybal is scheduled to introduce the ''Hispanic'' version of immigration reform, although the issue evokes such strong feelings that three of the nine Hispanic members of Congress have been unwilling to back it.
Among the Roybal proposals:
* Impose $10,000 civil fines against employers who violate recordkeeping requirements of federal labor laws. Employers such as garment manufacturers and farmers, which often hire illegals, fail to conform to other labor standards, goes the reasoning. But there would be no provision to make illegal the hiring of illegal aliens.
* Foster cooperation and trade agreements with Latin American neighbors to help slow illegal immigration.
* Reject the requirement for a worker to produce identification, and delete the provisions for farm guest workers, both of which are included in legislation known as the Simpson-Mazzoli bill, which passed the Senate and is awaiting House action.
The Roybal approach would avoid the chief Hispanic criticism - that sanctions against employers who hire illegals would inevitably lead to discrimination against workers who looked foreign. Moreover, according to an aide, Roybal would ''fight to the death'' any plan for a national identification system.
His bill would also give permanent resident status to illegal aliens who have lived in the US since Jan. 1, 1982.
Sen. Alan K. Simpson (R) of Wyoming, chief sponsor of the Senate version, holds that such legalization can only come if it is coupled with employer sanctions. Rep. Romano L. Mazzoli (D) of Kentucky, sponsor of the companion bill in the House, has taken a similar stand.
Meanwhile, the Senate will be delving into President's social-issues agenda, beginning with a proposed constitutional amendment to allow prayer in schools.
Senate majority leader Howard H. Baker Jr. (R) of Tennessee, in a move which not incidentally will please some conservatives, will bring to the floor later this month the amendment, which is said to have widespread public support but is certain to raise questions about church-state ties. As proposed by Senator Baker , the amendment would state, ''Nothing in this Constitution shall abridge the right of persons lawfully assembled, in any public building which is supported in whole or in part through the expenditure of public funds, to participate in nondenominational prayer.''