* CDU leader pokes fun at trans-gender people
* Joke was at expense of minorities, critics say
* Seen as Merkel's most likely successor
* Opposes gay marriage (Adds Kramp-Karrenbauer reaction in paragraph 9)
By Madeline Chambers
BERLIN, March 6 (Reuters) - The leader of Germany's ruling Christian Democrats (CDU), Annegret Kramp-Karrenbauer, drew sharp criticism from her Social Democrat coalition partners on Wednesday for poking fun at trans-gender people in a light-hearted carnival speech.
An outcry at the joke among her political foes and some media is the latest setback for Kramp-Karrenbauer, widely seen as the most likely successor to Angela Merkel as chancellor after taking over as conservative CDU leader in December.
Critics accuse Kramp-Karrenbauer of cracking a joke at the expense of minorities during Germany's six-day long pre-Lent carnival festivities, traditionally a chance for politicians to lampoon each other, dress up and drink a lot of beer.
"Clumsy jokes against minorities are the last thing that our society needs," said Social Democrat (SPD) Justice Minister Katarina Barley in a speech on Wednesday.
"Carnival should take aim at the powerful, at politicians, at companies, at banks but not at those who already have to fight," added Barley, the most senior politician to attack her. The conservative bloc and SPD share power in a grand coalition.
In a stand-up routine to an audience in the southern state of Baden-Wuerttemberg last week, Kramp-Karrenbauer mocked "latte macchiato" leftists in Berlin who want gender-neutral toilets.
"These (toilets) are for men who can't decide if they want to pee standing up or sitting down," she said in a video that was widely picked up on social media at the weekend. At the time she was dressed as a cleaning lady - her usual carnival costume.
Kramp-Karrenbauer, who was premier of the western state of Saarland before Merkel nominated her to become CDU general secretary a year ago, opposes gay marriage and has said marriage equality could lead to incest and polygamy.
Kramp-Karrenbauer hit back at critics of her joke during a speech on Wednesday before CDU members in the northern state of Mecklenburg-Vorpommern: "I have the feeling that today we are the most uptight nation in the world."
While the co-leader of the far-right Alternative for Germany (AfD) politician, Joerg Meuthen, defended her, other politicians and LGBT activists have condemned her.
"Another reason to fight to ensure this woman won't become chancellor," tweeted Dietmar Bartsch of the radical Left party.
"Is it so hard to make a funny joke without attacking minorities?" asked Free Democrat (FDP) Jens Brandenburg who branded the joke "embarrassing".
To some, Kramp-Karrenbauer, 56, is starting to look gaffe-prone. An Allensbach poll last month showed that only 47 percent of leading managers said they could imagine her as chancellor, with 49 percent saying they could not.
Last month, she was ridiculed for addressing a high-profile meeting of her conservative CDU party by calling them Social Democrats. German media jumped on the slip, with top-selling Bild calling it an "embarrassing mishap".
In a talk show last month, she also raised questions about her desire to succeed Merkel, who has said she will not run as chancellor in the 2021 federal election. Pressed on her ambition, she said as a responsible leader of the CDU, she may have to back a better candidate for the chancellorship. (Additional reporting by Reuters Television and Holger Hansen, Editing by William Maclean)
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