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The expression discrediting tactics refers to personal attacks for example in politics and in court cases. Discredit also means to not give the credit that was deserved, to cheat someone out of credit.
In politics 
The expression "discrediting tactics" in politics refers to personal attacks against a public figure intended to discourage people from believing in the figure or supporting their cause (see damaging quotations).
While a person may be insulted, and his political position subjected to question, the person so impugned has not been discredited until the allegations against him have been proved correct. One such discredit was the Impeachment of Bill Clinton. The allegations brought against the Chief Executive were damaging, but did not comprise discredit until enough substance arose from the testimony under oath of Paula Jones and Monica Lewinski to raise genuine questions of the credibility of President Clinton.
Roberts Rules of Order "Require appropriate censure whenever matters thought genuine are brought out that subsequently prove libellous, perjurious, or both... and where a person has repeatedly presented false or defamatory information for consideration, the person is subject to disciplinary action." See Perjury and Defamation.
Political debate often abuses public confidence by one candidate attempting to sway voters, not by logical argument on given issues, but by personal diatribe that does not directly bear on the matter at hand.
Accusations of adultery date back to the early 19th century. One famous example of this was the presidential campaign of 1884, in which Grover Cleveland was tarred by his opponents with having fathered an illegitimate child. A political catchphrase by his opponents was, "Ma, ma, where's my pa?" Cleveland won the election anyway, leading his supporters to add "Gone to the White House, ha ha ha".
Cleveland's defeat of his opponent, James Blaine may have been helped by another discrediting tactic used against him which seriously backfired, namely the assertion that Cleveland's party was that of "Rum, Romanism, and Rebellion" (the latter two referring to Roman Catholicism and the American Civil War). Cleveland's campaign also used the slogan, "Blaine, Blaine, James G. Blaine, The Continental Liar from the State of Maine" in reference to Blaine's discredited railroad deals.
Discrediting tactics are not just used against each other by opponents for office in democratic countries, but are also used in wartime between countries. In the middle of the 20th century, Soviet and British propaganda made popular the idea that Adolf Hitler had only one testicle (and was thereby less of a man).
American politics draws a line between "mud slinging" and defamation. The key issue is that mud slinging is not a form of perjury or libel. Politics also can include barratry where one opponent files frivolous litigation against the other, specifically to injure the opponent's reputation regardless that the case is groundless and may later be dismissed. By the time these facts can come to light, the voters have cast their ballots.
In court cases 
In the U.S. judicial system, discrediting tactics (called impeachment in that context) are the approved method for attacking the credibility of any witness in court, including a plaintiff or defendant. In cases with significant mass media attention or high-stakes outcomes, those tactics often take place in public as well.
Logically, an argument is held in discredit if the underlying premise is found, "So severely in error that there is cause to remove the argument from the proceedings because of its prejudicial context and application...". Mistrial proceedings in civil and criminal courts do not always require that an argument brought by defense or prosecution be discredited, however appellate courts must consider the context and may discredit testimony as perjurious or prejudicial, even if the statement is technically true.