Some 21.8 percent of respondents said this month that they would vote for the Social Democrats if general elections were held next Sunday, down from 24.5 percent who said so in November.
Other parties lag behind the Social Democrats in the poll.
The opposition Homeland Union-Lithuanian Christian Democrats moved into second place with 10.2 percent, up from 8.3 percent a month ago.
The Order and Justice party, a member of the ruling coalition, dropped to third place with 7.9 percent, down from 9.8 percent.
Further down the popularity list are the Labor Party, another member of the ruling coalition, with 7.8 percent, down from 8.2 percent, the opposition Liberal Movement with 7.2 percent, up from 6.2 percent, and the Lithuanian Peasant and Greens Union with 5.1 percent, unchanged from November.
Other parties would not clear the 5 percent hurdle required to enter the Seimas.
Vilmorus carried out the opinion poll between 5 and 14 December.