The campaign for a critical election in Germany's most populist state, North Rhine Westphalia, entered its final days today with German Chancellor Angela Merkel's conservatives edging ahead of their Social Democrat (SPD) rivals in opinion polls.

North Rhine Westphalia SPD Premier Hannelore Kraft had enjoyed until recently a comfortable lead in polls in Sunday's election, which is seen as a key test of the nation's political mood in the run-up to September's national poll.

But a batch of polls published over the last two days showed Merkel's Christian Democrats (CDU) overtaking the centre-left SPD in what had become a very close-run race.

The 13.2 million people who are registered to vote will face a ballot paper comprising 31 parties, including groups representing animal rights, the hard left and the far right, migrants and vegetarians.

After hitting 40 percent in March following the SPD's election of former European Parliament president Martin Schulz as its new national leader, support for the party had slumped to 31 percent according to a poll drawn up by public broadcaster ZDF and released yesterday.

At the same time, the CDU has crept past the SPD to reach 32 percent in the state, the ZDF poll showed, which was in line with another survey published by private TV station Sat 1.

Others surveys point to the election outcome as too close to call. A win for the CDU in North Rhine Westphalia would represent a big boost to Merkel as she enters the build-up to the September national election after solid wins in two other recent state elections.

A defeat for the SPD would be a major blow for the party and for Schulz, whose home state is North Rhine Westphalia.

"If the SPD comes second, then the downward spiral will be very difficult to stop," political scientist Oskar Niedermayer told dpa.

"That would mean Martin Schulz can bury his hopes for being chancellor," Niedermayer said.

- dpa