Backing for the SVP increased by about 2 percent between June and August polls, from 26.1 percent in June to 28 percent of voters in August.
The results strengthened the party's position ahead of all others in the poll released by gfs.bern research institute on Wednesday.
Source: SRG SSR/gfs.bern Wahlbarometer 2015.
The Socialist party (SP) remained second to SVP at 19.3 percent, while support for more centrist parties started to decline further.
The centre-right Liberals party (FDP) saw a drop from 17.1 percent to 16.9 percent and the centrist Christian Democrats saw a decline from 11.5 percent to 11.1 percent.
"The electoral barometer shows that the polarization of the electorate will probably increase again," said Chairman and Chief Executive Officer of gfs.bern, Claude Longchamp, in an analysis of the poll.
The Swiss People's Party, with its nationalist policies and support for immigration restrictions, remains popular at a time when the issue of immigration and asylum seekers is the biggest concern of voters.
Nearly half of all respondents - 46 percent - said in the latest poll that the issues of migration, foreigners and refugees were the most urgent issues that Swiss politicians must solve.
And 29 percent of respondents they thought SVP was the party most apt to handle such matters.
But controversy was sparked recently when SVP federal MP Christoph Mörgeli posted comments on his Facebook that were flagged as offensive and deriding towards refugees.
One post showed a photo of a boat full of refugees arriving at a quay,with Mörgeli adding a caption stating "the qualified staff is arriving".
The comments drew a wave of complaints online and Facebook ultimately removed his profile, saying that Mörgeli had contravened community standards.