BERLIN (Reuters) - A leader of the anti-immigrant Alternative for Germany (AfD) warned on Sunday that his party risked disintegrating if it failed to root out extremism as mainstream politicians called for intelligence services to monitor some of its members.

The AfD was founded in 2013 to oppose European bailouts but gained support in the past year as more than a million refugees arrived in Germany. It has recently suffered in the polls due to a power struggle among its leaders and a public row over anti-Semitism.

AfD co-leader Joerg Meuthen left the party's parliamentary group in the state Baden-Wuerttemberg last week after he failed to win support to expel AfD lawmaker Wolfgang Gedeon, who said Holocaust denial was a legitimate expression of opinion.

"It is an existential question whether we succeed in credibly distancing ourselves from extremism and anti-Semitism," Meuthen told Tagesspiegel am Sonntag. "If we succeed, we can go over 20 percent (of votes). If we don't, we will go under."

The right-wing AfD has seats in eight of Germany's 16 assemblies. Many commentators expect it to enter parliament in the 2017 federal election.

Denial of the Nazi Holocaust, in which six million Jews were killed, is a crime in Germany. The row has hit AfD ratings with an Emnid poll on Sunday showing support slipping one point to 10 percent, its lowest level since January.

It also has prompted calls for the domestic intelligence agency to monitor radical members of the AfD.

"The intelligence agency should keep a keen eye on the AfD and individuals from this party," Thomas Strobl, conservative interior minister of Baden-Wuerttemberg, told the Frankfurter Allgemeine Sonntagszeitung.

Regional intelligence agencies were already watching AfD lawmakers active in other radical groups, the newspaper reported.

"This has been established in some individual cases. There is a link to the right-wing and Islamophobic scene," Burkhard Koerner, head of Bavaria's intelligence agency told the paper.

Meuthen has clashed with the other AfD co-leader Frauke Petry over the Gedeon row as Petry backed those who had let him stay. Gedeon quit last week. Meuthen said Petry and he "don't have to love each other but we must work together pragmatically".

The two issued a joint statement at the weekend saying they wanted to work together in future.

(Reporting by Madeline Chambers; Editing by Raissa Kasolowsky)