Rabbi Dov Fischer of Irvine, California was once known as David Fisch, JDL Executive Director.

Rabbi Dov Fischer of the Young Israel of Orange County (a more right-wing breakaway from the Beth Jacob Congregation of Irvine) is known both as one of the more prominent rabbinic proponents of what I like to call "daas radio" (like daas Torah, except the "gedolim" are Dennis Prager, Rush Limbaugh, Sean Hannity, Michael Savage, etc.)  as an implacable foe of left-wing Modern Orthodoxy/Open Orthodoxy. A recent post of his on Cross-Currents merges his two enemies (the political liberals and the more liberal Orthodox) in a rather unique manner. Unlike most other complaints, which generally at least focus on actual halakhic issues, Rabbi Fischer begins his complaints with an attack against Rabbi Hanan Schlesinger ( a long-time resident of the Alon Shvut settlement in Judea and Samaria a.k.a. the West Bank), for acknowledging the basic factual existence of a military occupation in the West Bank and that Palestinians may suffer under it some.

I thought it was rather weird,  especially for a man living in Irvine to begin a broadside against people for not being Orthodox against a man living proudly in the Land of Israel. Moreover,  as far as I can tell, Rabbi Schlesinger is uncontroversially modern Orthodox in ritual/religious practice and merely happens to be a member of the religious peace camp, which as far as I can tell in his case means he supports coexistence, not dividing the land (the latter has apparently been held to be permissible in theory by Rabbi Soloveitchik, but I don't want to get into that here).  At any rate, an explanation for this weirdness may lie in his past.

The long and detailed biography on Rabbi Dov Fischer's website leaves out a very important biographical detail: in the 1970s, he was the executive director of the Jewish Defense League (JDL), at which time he went by the name David Fisch, which later became Dov Fisch, still later became Dov Aharoni (or Dov Aharoni-Fisch) and is now Rabbi Dov Fischer. Although Rabbi Fischer does not acknowledge his past explicitly, he does not do a particularly great job of hiding it, hawking on his personal website the following book he wrote when he went by Dov Aharoni Fisch.  "Jews for Nothing" is mentioned on a website of the Jewish Task Force, a Kahanist organization, as having been written by David Fisch, former JDL head. Elsewhere on their forum they make an explicit connection on a thread that David Fisch AKA Dov Fisch AKA Dov Aharoni AKA Dov Fischer are all the same person.  It is probably unwise to rely on random forum posts alone to definitively establish that Rabbi Dov Fischer used to be JDL head David Fisch, so I have compiled some further corroborating evidence below which I believe definitively establishes it.

• Rabbi Fischer's LinkedIn account discusses how he went to Columbia University for his undergraduate degree and was "Elected as one of two College representatives to University Senate." It later discusses how he received semikha at RIETS  (Yeshiva University Rabbi Elchanan Theological Seminary), the flagship yeshiva of Modern Orthodoxy; although no date is given on Linkedin, his official biography states he received semikha in 1981.
• On page four of the March 23, 1977 issue of the Columbia University Spectator, Dov Fisch writes in to complain about Columbia dismissing the JDL from student offices, stating "As an alumnus of Columbia, as a former University Senator, and as the founder of Columbia JDL, I have continued to take an active interest in the chapter's progress, despite the fact that I am now studying uptown for my rabbinical ordination." Lest there be any concern about which uptown rabbinical school he meant, he later states in the article "At the end of the first term, JDL chapter-chairman Danny Simckes decided to drop out of the JTS-Columbia joint-program, having been disgusted by the religious compromising and spiritual fraudulence offered by the "Conservative" seminary, often referred to as the "Jewish Theological Cemetery."" Furthermore, an ad placed in the November 3, 1976 issue of the Columbia Spectator  makes it clear that Dov Fisch was studying at YU.   
  
  It is worth noting that although in the article linked above, he only claims to have been the founder of the Columbia JDL, it is clear from this article from the Jewish Telegraphic Agency from July 16, 1974 that he was in fact the executive director of the JDL as a whole, "David Fisch, a 22-year-old Columbia University student, said today he had resigned as executive director of the Jewish Defense League."  Note that while he resigned then as the executive director, he maintained involvement with the JDL.
•  He was arrested on March 30, 1975 with 21 other JDL members for throwing bottles through windows at the Soviet Mission to the UN (although he denied it)
• In a June 8, 1976 JTA article, after 3 JDL members were arrested by the FBI with a large quantity of black powder (used for making explosives), he is said to the be "JDL associate director" and claimed the FBI planted the powder. Note that five JDL members were eventually convicted in December on charges relating to these arrests.
• On his personal website, Rabbi Fischer states that after receiving semikha but before making aliyah in 1985 and founding a new settlement in the Samaria area of the West Bank, he (among other things) served as national executive director of the Likud Zionists of America. That organization, better known in those days as Herut USA, can be seen in the 1985 edition of the American Jewish Yearbook (pg. 370 of the full yearbook, 39 of the PDF) to be led by Executive Director Rabbi Dov Aharoni-Fisch. In an article from 1983, he is referred to simply as Dov Fisch.
• The David/Dov Fisch discrepancy can incidentally easily be explained in that David is his legal name, Dov his Hebrew name. The person-data aggregating website Spokeo gives a "David Ber Fischer" of age 64 living in Irvine, CA, giving as his aliases Dov Ber Aharoni and David Fisch. That this is Rabbi Dov Fischer can be seen by relatives Linda (Yellin) [his ex-wife], Yael (his daughter) and Ellen (his current wife), in addition to having also resided in Louisville, as Rabbi Fischer's bio proudly states that he "has been named by three different Governors of the Commonwealth of Kentucky — Gov. Brereton Jones, Gov. Paul Patton, and Gov. Ernie Fletcher — as an Honorary Kentucky Colonel." Moreover, the Columbia Spectator archives alternate between calling him David Fisch and Dov Fisch.

I believe the above makes Dov Fischer's previous JDL life incontrovertibly clear. 

I will end on an interesting and somewhat irrelevant note, but one that may nevertheless bring schadenfreude-style joy to some of us. One of his children, YJ (Yael Julie) Fischer, is a liberal Democrat. Her marriage to Daniel Baum Schwerin in November 2017  made the New York Times. While Mr. Schwerin appears to be Jewish, the ceremony was performed by Lisa Roman, a friend of the couple who became a "Universal Life minister" in order to perform the ceremony.  In the wedding profile, they discuss how she and her husband (who incidentally was the director of speechwriting for Hillary Clinton's 2016 campaign and her principal collaborator on her "What Happened" memoir) met in 2005 while working on now-Senator Tim Kaine (D-VA)'s campaign for Governor in Virginia Beach-as a grandchild of 2 secular Jews who met at a victory party for Harry Truman, I find that endearing.