Sen. Christopher J. Dodd (D-Conn.) told reporters today that he opposes the Reagan administration's proposal to send U.S. Marines here to ensure a peaceful withdrawal of PLO guerrillas.

Sen. Carl Levin (D-Mich.), accompanying Dodd on a Middle East fact-finding tour, disagreed, saying he supports the administration plan.

In a preview of Senate debate over the plan, Dodd disputed Levin's suggestion at a news conference that Marines could be landed after all factions had agreed not to fire on them.

"There are more factions in Lebanon than combinations to a Rubik's Cube," Dodd said. "There are too many factions in Lebanon to guarantee anything."

The Marines, he said, "will be a target."

Dodd said he would explain his position further during confirmation hearings Tuesday for Secretary of State-designate George P. Shultz before the Senate Foreign Relations Committee, of which he is a member. Levin is a member of the Senate's Armed Services Committee.

The senators' press conference was punctuated by some of the heaviest shelling since Tuesday between Israelis and the PLO during the latest violation of the three-week-old cease-fire.

Both senators said they believed that the PLO and all foreign troops, with the exception of the 7,000-man U.N. peace-keeping force, should be withdrawn from Lebanon as soon as possible.

In addition to a 60,000-member Israeli force and about 6,000 PLO guerrillas, there are 40,000 Syrian troops in Lebanon.

The senators' comments followed a long lunch with the rightist Christian Phalangist leader Bashir Gemayel, who yesterday publicly criticized the plan to bring U.S. Marines here. Gemayel said PLO leader Yasser Arafat would only use their presence to prolong the stay of his guerrillas, a position that Dodd agreed with.

Levin argued, however, that the risk of the Marines becoming targets or being drawn into Lebanon's factional fighting would be reduced if "all factions invite us in." It would be worth the "limited risk," Levin added.

Both senators said they had received assurances from Israeli Prime Minister Menachem Begin during their stop in Israel that the blockade of food shipments into West Beirut would be lifted. Checks at the two Israeli-controlled crossing points showed, however, that food was still not being allowed in for the general population.