BERLIN (Reuters) - In past years, Angela Merkel has been feted like a superstar at annual meetings of her Christian Democratic Union (CDU) party, earning thunderous ovations for defending German interests in the euro crisis and facing down Vladimir Putin over Ukraine.

File photo of German Chancellor Angela Merkel attending the Christian Social Union (CSU) party congress in Munich, Germany November 20, 2015. REUTERS/Michael Dalder/Files

But a CDU congress in the southwestern city of Karlsruhe next week is shaping up to be a very different affair. Under intense pressure from conservative allies to reduce the flood of refugees into Germany, the 61-year-old chancellor faces the biggest test of her authority from within the party in years.

The influential youth wing of the party has openly defied her in the run-up to the glitzy two-day event by demanding she agree to an “Obergrenze”, or cap on the number of asylum seekers Germany accepts - a step she has repeatedly rejected on the grounds it would be impossible to enforce.

Her Bavarian allies, the Christian Social Union (CSU), have been pressing for a cap for months, and even some of Merkel’s own ministers are lobbying openly for a tougher stance from the chancellor, who marked 10 years in office last month and must decide by next autumn whether she will seek a fourth term in 2017.

“Merkel has never endured such sharp criticism from within her own ranks since becoming chancellor,” read a front-page editorial in conservative daily newspaper Frankfurter Allgemeine Zeitung on Monday. “Under no circumstances can she allow the congress to approve a resolution on refugee policy that includes the word ‘Obergrenze’.”

Two recent developments are working in Merkel’s favor ahead of the meeting in Karlsruhe, a city which sits near the border between Baden-Wuerttemberg and Rhineland-Palatinate, two of three German states that will hold elections in March.

First, the number of migrants arriving in Germany has slowed significantly since late November, largely because of colder weather which has made it more difficult for refugees to travel from Turkey to Greece and then up through the Balkans.

“It’s too early to declare a change in the trend but it is positive,” Interior Minister Thomas de Maiziere said on Monday.

Also, a sharp months-long slide in support for Merkel and her conservative bloc appears to have come to a halt.

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An Emnid survey last weekend put the CDU/CSU on 37 percent, up from 36 percent a month ago, and still 12 points ahead of the rival Social Democrats (SPD). A separate poll from Infratest dimap showed Merkel’s popularity rising 5 points to 54 percent.

“I see a lot of support in the party for Angela Merkel’s course,” CDU General Secretary Peter Tauber said at the weekend, dismissing suggestions her authority was waning.

‘CATASTROPHIC MOOD’

But some CDU members described the mood in the party as abysmal.

For the first time in years, off-the-record conversations with lawmakers in Berlin are littered with criticisms of Merkel, echoing the era before she became chancellor when a cabal of conservative men worked behind the scenes to undermine the protestant pastor’s daughter from communist East Germany.

Last week, former Saxony justice minister Steffen Heitmann became the first prominent member of the CDU to announce he was leaving the party. In a letter to Merkel which was leaked to the media, he said he had never felt “so foreign in my own country”.

The atmosphere could not be more different than it was back in 2012, when at a CDU congress in Hanover, Merkel was re-elected party leader by 98 percent of delegates, a score so high that German reporters jokingly likened it to the sham elections of East German leaders during Merkel’s youth.

“The mood among conservative members of parliament is really catastrophic right now,” said one senior CDU lawmaker, declining to be named. “Merkel is totally isolated.”

“She needs to wake up,” said another top ranking party member. “Merkel’s solution to this crisis depends on the goodwill of people like (Russian President Vladimir) Putin and (Turkish President Tayyip) Erdogan. It simply won’t work.”

A close aide to the chancellor, speaking on condition of anonymity, played down the extent of the discontent, estimating that only a third of party members were really in favor of a tougher course, through caps, border closures or more radical measures.

Merkel has resisted such steps, arguing that the influx must be tackled outside Germany, through negotiations to resolve the war in Syria and by encouraging neighboring Turkey to improve conditions for refugees there and convincing European partners to accept quotas of asylum seekers.

“Merkel will not budge on this,” the aide said. “If there really is a majority at the congress for caps on refugees, this would point to a problem at the top, but we don’t expect this.”