Washington — If President Reagan's decision to send American troops back to Beirut came to a vote, Congress would - with considerable reluctance - approve.
This is the conclusion drawn by a number of congressmen and staff aides on Capitol Hill in the light of President Reagan's assurances that American participation in a multinational peacekeeping force would not involve police action and would be for a limited period.
Congressional hesitation, reservations, and fears are such, however, that should American troops suffer casualties in Beirut, many senators and congressmen would immediately reconsider their support.
There is already more criticism of the President's decision in Congress this time around than there was when he sent marines in a month ago, for only 16 days , to oversee the departure of Syrian and PLO forces from Beirut. On that occasion, the marines left Beirut earlier than had been anticipated. On this occasion, the commitment is more open-ended, the planning more hurried, and the functions of the force not yet precisely defined.
But the reintroduction of American forces is designed to fulfill the same overall goal as the earlier deployment of US marines there: to establish a highly visible military ''presence'' that will help bring stability to a country racked with violence. In addition to that goal, the administration hopes to use the momentum created by the move back into Beirut to work toward the withdrawal of all foreign forces from Lebanon and toward reestablishing the credibility of President Reagan's plan for a resolution of the Arab-Israeli conflict.
As seen from here, Israel's takeover of west Beirut and the subsequent massacre of civilians threatened to destroy not only the credibility of American guarantees for the protection of Beirut and its noncombatants but also threatened to destroy the credibility of the entire Reagan peace plan.
The administration would have preferred to help send in a United Nations peacekeeping force - without the commitment of American troops - but it was finally decided that such a force would take too long to form and would be unacceptable to the Israelis. The administration is still working, however, toward the goal of having a UN force replace the multinational force of American , French, and Italian soldiers now on their way to Beirut.
The Congress is not required to act on the issue unless the United States' contingent in that force is to be deployed for more than 60 days.
Clement J. Zablocki, the Democrat from Wisconsin who heads the House Foreign Affairs Committee, seemed to sum up much of the sentiment in the Congress when he said, ''We are a little concerned that this time our military forces will be in greater jeopardy. The last time they were stationed in a very limited area, and the chances of their suffering casualties were minimal.''
Mr. Zablocki told the Monitor that he thought a bare majority - about 55 percent of Congress - supported the presidential decision to send troops at this point.
One Senate staff specialist said that behind the feeling of some senators that Reagan had no choice but to accede to the Lebanese request for troops was a feeling that the US had been misled on a number of occasions by Israeli Prime Minister Menachem Begin as the Israelis moved deeper into Lebanon. It was not clear, however, whether a majority of senators and congressmen would favor sanctions against Israel - such as an aid cut - if the Israelis refused to leave west Beirut. One key staff aide suggested that at this point Congress would simply refuse to vote requested aid increases for Israel and would balk at expanding the US-Israel relationship in defense cooperation and research and development.
Meanwhile, one official, Assistant Secretary of State Nicholas A. Veliotes, argued that the marines going into Beirut would actually face a less potentially dangerous ''environment'' this time than the last time they went in. In an interview with ABC television on Sept. 21, Mr. Veliotes noted that a large number of PLO and Syrian fighters had been removed from Beirut. But beneath the official pronouncements, there is considerable concern among many government officials and military experts that the situation this time could not only be more difficult but also more dangerous.
This time, the multinational force is charged with a more difficult task: to ensure that the Lebanese government is ''strong enough to secure its own borders and its own cities,'' as Defense Secretary Caspar Weinberger puts it.
According to US officials, the marines this time will not be confined to Beirut's port area. Instead, they likely will move farther south into the city, taking up positions (as a senior official noted) ''between two population groups that have had an historic antagonism toward each other.'' Aside from the Lebanese factions, there may be remnants of the PLO to contend with as well.
''There's bound to be PLO resistance left,'' said William Taylor, director of political-military studies at Georgetown University's Center for Strategic and International Studies. ''It's probably very disorganized, but it doesn't take much when you're trying to upset and destabilize a situation. You could do it with a squad.''
''There's always the chance of some kind of contact,'' says Dr. Taylor, but in order to ensure that the marines are not drawn into internal Lebanese conflict they will ''have to be kept on a very tight string.''
Whether or not that is possible, however, is highly problematical, Pentagon officials acknowledge.
''I was surprised we got out of there with our whole skin the last time,'' said one officer. ''So I don't know that it's going to be a more dangerous situation. It was dangerous to start with.''
The 800 marines scheduled to go to Lebanon are part of an 1,800-man amphibious force now in Italy and assigned to the US Sixth Fleet. Their rules of engagement will be the same as during their earlier tour in Beirut - they will have light arms, and be able to defend themselves if fired upon, according to the Pentagon.
Realizing that the task this time is more difficult, officials are not saying exactly how long the marines will be there. Last month, their stay was limited to 30 days, but they left after 16 days. Under the War Powers Act, Congress would have to approve any deployment of more than 60 days.