Plea bargaining in the United States
The constitutionality of plea bargaining was established by "Brady v. United States" in 1970, although the Supreme Court warned that plea incentives which were sufficiently large or coercive as to over-rule defendants' abilities to act freely, or used in a manner giving rise to a significant number of innocent people pleading guilty, might be prohibited or lead to concerns over constitutionality. "Santobello v. New York" added that when plea bargains are broken, legal remedies exist.
Plea colloquy
Pursuant to the Sixth Amendment to the U.S. Constitution, a criminal defendant has the right to be represented by an attorney during a plea colloquy; failure of the state to provide an attorney to an indigent defendant during such proceedings is grounds for an appeal. It is possible - but very difficult - for a defendant who "is" so represented to have a plea thrown out due to ineffective assistance of counsel. The defendant must make a positive showing that but for the erroneous advice of counsel, he would not have chosen to plead guilty.