STV differs from all other proportional representation systems in actual use in that representatives are directly elected by the voters, and candidates of one party can be elected on transfers from voters for other parties. Hence, the use of STV may reduce the role of political parties in the electoral process and correspondingly partisanship in the resulting government. Unlike proportional representation systems employing party lists, voters in STV are not explicitly constrained by parties even when they do exist; voters may ignore candidate party labels and mix their preferred candidate rankings between parties. Similarly, candidates may achieve electoral success by obtaining a quota of voters not generally within their own party, perhaps by winning transfers from moderates or by championing a specific issue contrary to party doctrine. STV advocates argue that, by requiring a candidate to appeal to the supporters of other candidates for their second and further preferences, it reduces adversarial confrontation, and indeed, gives a substantial advantage to candidates that broaden their appeal by being not only collegial but as open-minded and flexible in their principles as they can manage. STV detractors see that as a flaw, arguing that political parties should be able to structure public debate, mobilize and engage the electorate, and develop policy alternatives.