Chancellor Angela Merkel's conservatives rallied behind her bid for a fourth term as German leader on Monday, laying aside months of disunity to help head off a surge in support for the rival Social Democrats (SPD).

But at a press conference following a two-day meeting of Merkel's Christian Democrats (CDU) and their Bavarian-based allies the Christian Social Union (CSU), the chancellor ruled out imposing a ceiling on the number of refugees Germany will take in if she wins elections in September.

Merkel's statement underlined the deep division which has opened up in her CDU-led bloc over her handling of the refugee crisis that engulfed the nation about 18 months ago.

"I do not intend to change the position here," the head of Europe's biggest economy said after the CSU agreed to back her as their chancellor candidate in the elections, which Merkel said would be the most difficult the nation has faced.

The attempt by Merkel and CSU head Horst Seehofer to present a united front came after the SPD last week surprised the nation's political establishment by nominating the former European Parliament president Martin Schulz to lead the party's campaign.

While the 61-year-old Schulz is a well-established figure on the European political stage, he remains somewhat unknown in Germany, leading analysts to suggest that his outsider status has added to his current political appeal.

Relations between the CDU and CSU have been strained since Seehofer spearheaded criticism of Merkel's decision in September 2015 to open Germany's borders to allow hundreds of thousands of asylum seekers into the nation.

Since then, Merkel, 62, has repeatedly rebuffed Seehofer's demand that she impose a ceiling on the number of refugees.

Andreas Vosskuhle, who heads the Constitutional Court, said last year that placing a limit on the number of refugees entering the nation was contrary to the constitution, which calls on the country to provide refuge for those escaping persecution.

The CDU-CSU is, however, also facing a major challenge from the rise of a new right-wing populist party, the Alternative for Germany (AfD), which has used the nation's refugee crisis to mobilize support.

Pollsters say the tensions in the CDU-CSU over Merkel's refugee stance has contributed to a sharp fall in support for both parties.

In his comments, Seehofer sought to play down differences over his demand for an upper limit and instead praised Merkel as "an excellent chancellor - both nationally and internationally."

But underscoring the pressure on Merkel, a survey drawn up by pollster INSA for the daily Bild and released on Monday showed the SPD defeating the chancellor's conservatives if an election was held today.

While the SPD would garner 31 per cent of the vote, Merkel's CDU-led bloc would come in behind with 30 per cent, the poll said. The AfD was in third place with 12 per cent of the vote.

Merkel's CDU backed her as its candidate for chancellor at a party conference in December, but the CSU had not declared its support until this week's meeting.

The build-up to the national election in September will be accompanied by a series of unpredictable state elections, including in Germany’s biggest state North Rhine Westphalia in May.

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