BERLIN: The anti-immigrant Alternative for Germany (AfD) is poised to win almost 20 percent of the vote in a state election and match the ruling Social Democrats in another this month, highlighting the threat to mainstream parties from the migrant crisis.

The elections in Saxony Anhalt, Baden-Wuerttemberg and Rhineland-Palatinate states are Germany's first since last May and will serve as a litmus test of popular feeling on the crisis over the influx of 1.1 million migrants into Germany last year.

A poor showing by Chancellor Angela Merkel's conservatives in these elections would increase pressure on her to reverse a disputed open-door policy towards migrants, 1-1/2 years before a federal election when she is likely to seek a fourth term.

A poll for broadcaster ARD put support for the AfD in the eastern state of Saxony Anhalt at 19 percent, making it the third most popular party behind Merkel's Christian Democrats (CDU) and the Left.

In Baden-Wuerttemberg, a wealthy southwestern state that hosts luxury carmakers Porsche and Daimler, the AfD was seen winning 13 percent of the vote - good for joint third place with the Social Democrats (SPD), who currently govern together with the Greens. The Greens led in Baden-Wuerttemburg at 32 percent, with the CDU next at 28 percent.

The AfD's weakest showing was seen in neighbouring Rhineland Palatinate at 9 percent, but this would still make it the third-strongest force in the state behind the CDU and SPD.

All three states go to the polls on March 13 and have a combined population of 17 million, over a fifth of Germany's 81 million.

The AfD's rise has chipped away at support for Germany's established parties and may complicate their efforts to form stable coalition governments.

The AfD has gained ground as many voters have turned against Merkel's decision to open Germany's borders to refugees fleeing war and deprivation in the Middle East and Africa.

Merkel defended her policy on Sunday and rejected any limit on the number of refugees allowed into the country, despite calls from within her coalition government for restrictions on the numbers allowed to arrive.