BERNIE SANDERS enters today’s Democratic primary election in Wisconsin with the wind in his sails following his recent triumphs. But if he wins it will be only his fifth victory in a primary election; all his other successes, including his most recent, have been in states that hold caucuses, a more complex system for choosing delegates to the party’s national convention. Caucuses are local meetings of party loyalists that have much lower participation rates than primary elections. They also tend to be dominated by activists. In his recent caucus wins, Mr Sanders’s supporters have propelled him to huge margins of victories: 78% to Hillary Clinton’s 21% in Idaho, for example, and 73% to 27% in Washington state (on the Republican side, Ted Cruz has also had a better success rate in caucuses).

But the process for allocating delegates through caucuses does not end with the tally of votes on election night, and can produce some surprising twists. In Nevada, for example, Hillary Clinton was adjudged the winner in the caucuses held on February 20th, taking 53% of the vote to Mr Sanders’s 47%. But on April 2nd Nevada’s Democrats held county conventions, which determine whose supporters should attend Nevada’s state convention in May, where more delegates to the party’s national convention in July will be allocated. Back in February Mrs Clinton won Clark County (Las Vegas and its suburbs) by ten-percentage points, but Mr Sanders’s supporters turned out to the county’s convention in greater numbers and more of his voters will now attend May’s state convention. His campaign now thinks they have whittled back Mrs Clinton’s lead in Nevada’s pledged delegates and might actually come out ahead after the state convention.

So Mr Sanders has been piling up delegates through the caucus system, though Mrs Clinton remains some 2.4m votes ahead. Happily for Mrs Clinton there are only two caucuses left in the Democrats’ nomination calendar.