Germany's far-right Alternative for Germany (AfD) party appeared to have increased their support in elections in Saxony and Brandenburg, according to early exit polls on Sunday.
Some 5.5 million people voted in the eastern German states. In Saxony, the AfD scored 27.5 percent - up from 9.7 percent five years ago, reported the broadcasters ARD and ZDF.
It won between 22.5 and 24.5 percent in Brandenburg state, compared to 12.2 percent in 2014.
As well as lambasting asylum-seekers, the AfD has capitalised on resentment about a lingering east-west wealth gap since the 1989 fall of the Berlin Wall.
Eastern Germany is home to several of the AfD's most extremist leaders, among them Bjoern Hoecke, who has labelled Berlin's Holocaust memorial a "monument of shame".
His ally, former paratrooper Andreas Kalbitz, 46, is the top candidate in Brandenburg where the AfD has been matching the governing Social Democratic Party (SPD).
In Saxony, the AfD has slipped back several points behind the centre-right Christian Democrats (CDU) of chancellor Angela Merkel.
But even if the AfD emerges as a big winner in both states, the other major parties are expected to form coalitions to achieve majorities.
However an election debacle for either Merkel or the coalition partner the SPD would pose another threat to their union.
Merkel has pledged to step down when her term ends in 2021 but regional election upsets could speed up her government's demise.
Poor results for the SPD, who are in disarray after several election defeats, would boost internal critics who want the party to leave Merkel's government quickly.