The party, known as Die Linke in German and made up of former communists and defectors from the mainstream centre-left Social Democrats (SPD), is the top choice for 14 percent of voters, a poll in weekly Bild am Sonntag showed.
The surge of 5.3 percentage points puts them ahead of the environmentalist Greens and the pro-business Free Democrats (FDP), which traditionally have played the role of kingmaker to help the SPD and the conservative Christian Democrats (CDU) form coalition governments.
The FDP was on 10 percent, up 0.2 points, and the Greens were on nine percent, a rise of 0.9 points. German Chancellor Angela Merkel's CDU scored 36 percent, up 0.8 points. The rise of the Left Party appears to be mostly at the expense of the SPD, which together with the CDU forms the current ruling coalition. Support for the Social Democrats slid 7.2 points to 27 percent.