According to Wikipedia, a party leader is the most powerful official within a political party. I think this statement is debatable. Party leaders in recent history have had varying degrees of power and influence over their own party members, let alone the wider political system.
Writing in The Atlantic during the election campaign, @jon_rauch expressed strong opposition to the conventional view of party leadership.
The very term party leaders has become an anachronism. … There no longer is any such thing as a party leader. There are only individual actors, pursuing their own political interests and ideological missions willy-nilly, like excited gas molecules in an overheated balloon. …
This is not only a problem of leadership and individual agency, but also a question of the nature of the political party as a viable system with collective agency and intelligence. Rauch continues
The political parties no longer have either intelligible boundaries or enforceable norms.
The relationship between the politician and the party has always been problematic – consider Winston Churchill who changed party allegiance twice before becoming party leader. But the root cause of this problem is unclear.
Political parties are what things look like when you put politicians in charge.
— David Allen Green (@DavidAllenGreen) August 3, 2016
@DavidAllenGreen or they are machines for turning people into politicians
— Sean Owen-Moylan (@SeanOwenMoylan) August 3, 2016
Jonathan Rauch, How American Politics Went Insane (The Atlantic, July 2016)
Wikipedia: Party Leader (retrieved 4 Feb 2017)