The process for selecting presidential candidates is under attack. Politicians ranging from liberal Democrats to Trump Republicans don’t like it.

Some candidates, unfamiliar with party rules, have felt the process put them at a disadvantage.

The Republican and Democratic parties are not government organizations, and, under national party guidelines, each state party is free to set its own candidate selection rules. Traditionally, they have only allowed the party’s registered voters to participate.

Some “open” state primaries allow participation by voters not previously registered as party members. While state caucuses are open only to registered members, increasingly registration may take place shortly before the caucus or primary.

Bernie Sanders or Donald Trump, candidates running as “outsiders,” can find these rules make their races more difficult. Sanders, who had previously run for Congress as an independent, likes states allowing “open” selection of the Democratic candidate by non-Democrats.

Sanders’ supporters dislike the Democrats’ “superdelegates,” who don’t go through the election process and have a vote of their own at the national convention. They are awarded this right as government officials or party leaders.

The Democrats distribute convention delegates proportionately to the actual vote in a state. That lengthens the selection process, but it allows an important role for later voting states and, possibly, a more thoughtful course to the nomination.

Maine Democrats are leading the charge nationally to require a state’s superdelegates to vote in the same proportion as the primary or caucus participants voted. In other words, they still would get a seat at the convention thanks to their office, but lose their right to independent judgment.

If that rule were in effect this year, by last weekend, Sanders would have had 46 percent of the delegates instead of 40 percent, likely not enough to make a difference in the ultimate outcome.

Why do Democrats use superdelegates? Because voters in primaries or caucuses may not be representative of the party as a whole. Party officeholders, whose reelection may depend on who’s at the top of the ticket, are given special status.

Otherwise, something like the current problem in Britain may occur. There, newly registered Labor Party voters recently picked the party’s leader in Parliament, but he has only a handful of supporters among the members he is supposed to lead.

Superdelegates are a brake on the possibility of a takeover by an ideological minority or voters with little interest in the party. In that way, the superdelegates are meant to play, to a limited extent, a role reminiscent of the now extinct party bosses.

Clinton lined up broad superdelegate support early. Only an insurgent Sanders dared challenge her, gaining surprising success, which has encouraged him to step up his attacks on Clinton, even if that helps Trump.

Contrast this process with the Republicans. They have almost no superdelegates, allowing more power to enthusiastic newcomers, like the Trump supporters. Under GOP rules, states may distribute delegates on a winner-take-all basis, somewhat proportionately or some of each.

Trump has claimed at times that the GOP process is “rigged” or “crooked.” He believed that winner-take-all rules and the use of caucuses rather than primaries were designed to harm his candidacy. Not only were those charges untrue, in the end they didn’t matter.

If the GOP had superdelegates who lined up early behind a single candidate, as Democrats did for Hillary Clinton, they might have blocked Trump. Instead, 17 candidates split the party’s vote in the early going. Democrats might take heed of the GOP experience without superdelegates.

If, say, Jeb Bush had proved to be an attractive candidate with broad support from the outset, the result might have been different. In early contests, Trump managed to look like a big winner while capturing only slightly more than 20 percent of the vote in the huge field.

The problem now is probably less the process than the candidates. The parties seem to be having buyer’s remorse about their likely picks, and some voters probably believe that a different process would have produced a different result.

The Republican attitude toward Trump, at least at the moment, is mostly unenthusiastic or bewildered. Trump won as the clear alternative to a crowd of candidates, not because the GOP strongly favored him.

The beneficiary of broad backing, Clinton probably could recover party support more easily than the highly controversial Trump if she reveals a more honest and even contrite side. If she cannot deal better with her delay in turning over official emails from her home computer, she could remain vulnerable, losing her edge on Trump.