Learn English – the role of “every” in idioms like “every so often”

This is a nice story.

As already noted, the chandler is originally an occupation centred around the production and commerce of candles. The name itself comes from the French chandelier, which in addition to designating the candelabrum itself, used to designate a person whose trade it was to make and sell candles.

Since the fabrication of candles also yields soap as a by-product, most chandlers were also in the business of producing and selling soap.

Ships of course with always a proportion of seamen on the watch were natural consumer of candles and they needed to replenish their stocks every now and then throughout their sailing itinerary. There was indeed no compelling reason to purchase the whole stock of candles needed for a given journey in the home port. On the contrary, well advised shipowners would take advantage of the differences of prices for candles and other products in the home port and in the ports of call in order to level down the total expense.

They would therefore enter in global procurement deals with large volume chandlers located in the various ports of call and open a line of credit with each of them.
They would also have their seamen benefit from the deals not only to increase the purchased volumes and get a better deal but also as a deal sweetener towards the crew - possibly lower wages compensated by additional benefits.

Yet for the seamen that actually was a big advantage because they knew were to get stuff in each port and they would not need to change currencies (at least for these kind of supplies ;-). All they had to do was to make sure they took their allowance of each permitted product.
Besides, even if they'd drink up their wages, the quartermaster would still be assured that they'd have received basic commodities from the chandler's stores.

The structure of the actual financial arrangements was such that chandlers were naturally drawn into extending the range of offered items to whatever ships and individual seamen actually needed, much beyond the initial speciality of candle making.

This is why the profession of chandler gradually shifted from just candle and soap making to that of one stop shops for merchant ships in the 18th and 19th century.

In the wake of previous centuries' English naval supremacy one might say.

^{They supply the crew's food, ships maintenance supplies, cleaning compounds, paint, rope, et cetera. The advantage of a ship's crew using a chandler is that they do not have to find stores in the town they have landed in, nor hold that local currency - assuming they are let out of the dock compound by the immigration authorities. Typically, the ship owner has a line of credit with the chandler and is billed for anything delivered to the crew of his ship. The chandler is supplied by merchants local to his location.
Their distinguishing feature is the high level of service demanded and the short time required to fill and deliver their special orders. Because commercial ships discharge and turn around quickly, delay is expensive and the services of a dependable ship chandler are urgent. Because these contracts are so lucrative, the practice developed among ship chandlers to rebate to ships' officers approximately 5% of the invoice price of the ships' stores billed to the vessel, to encourage repeat business.}

As others have said, it's a general term conveying the idea of one by one.

I'd like to add, though, that at turns may have been used because there are two things that could be said to have come one after another: the candidates, and their gaffes. So, in no particular order: Bachman was perceived as ridiculous, Cain was perceived as self-destructive, Huntsman was perceived as ineffectual, etc.

The author probably didn't want to be so specific, partly because the correlation between candidates and gaffes isn't as clear-cut as I've indicated here, and partly because a single candidate may have compounded the said gaffes atop each other in a series of political missteps. So, an efficient way to convey this sentiment is to state the generality:

His declared rivals were at turns ineffectual, ridiculous, or self-destructive...

leaving the reader to work out the details of who stumbled when, and how each one stumbled. Meanwhile, the author can move straightaway to the main point of the sentence, which is that Romney had "a fairly easy, if occasionally fraught, path to the nomination."

I'd conclude that at turns is not idiomatic, but instead an efficient way to describe a sequence of people independently making a series of mistakes.