Why are the current US Presidential election primary campaigns noticeably different from previous campaigns?

The success of insurgent, populist and 'outsider' candidates so far in the 2016 primary cycle, for both the Democrats and the Republicans, can be seen both as a result of technical and legal changes to campaigning rules, and as a more profound change in the ideological and political demands of primary voters.

Donald Trump's lead in the polls so far for the Republican nomination is primarily a result of the way his campaign has harnessed the anti-politics and anti-political correctness mood, a phenomena existing in many Western democracies. Although his arguments are rarely able to stand up to close scrutiny, his supporters seem impervious to this. The demographic of these voters is primarily white, less educated, poorer men - the very demographic that has fared worst from globalisation, affirmative action policies and greater equality between sexes and races. Anti-immigrant, anti-politician rhetoric chimes well with such voters.

The success of Mr Trump, and his potential staying-power, can also be attributed to multiple relaxations of campaign finance rules from recent Supreme Court rulings. Mr Trump is financing his own campaign, while other outsider, high-polling candidates such as Ben Carson and Carly Fiorina are able to stay in the race longer, despite their smaller fundraising bases. 

Ultimately the effect of such candidates will be important. It is unclear whether Mr Trump will remain in the race; even if he does, if another candidate is able to attract support from across the party as a 'unity' candidate, they may well be able to defeat him. The aforementioned changes to campaign finance rules, though, make this scenario less likely, as it depends on the majority of candidates dropping out. The eventual winner will most likely be someone able to both harness the populist rhetoric of Mr Trump and to secure the backing of the party establishment and big business donors. Marco Rubio, the Florida Senator, as a relative outsider but also a relative moderate (in comparison to Mr Trump) could well be that candidate.