Carly Fiorina is spending a lot of time making a key argument against the likely Democratic nominee for the presidency in 2016. Hillary Clinton, Fiorina told George Stephanopoulos in making her official announcement Monday, "clearly is not trustworthy, about a whole set of things." Hillary Clinton is not trustworthy, she said on "Fox and Friends" in April. "In effect, @HillaryClinton told us to trust her," Fiorina tweeted in March. "Nothing in her track record suggests we should do so."

Americans seem to agree. Our Chris Cillizza outlined recent poll numbers showing that people don't consider Clinton to be trustworthy.

But there's a possible reason for that. Like her favorability rating, Clinton's "trustworthiness" seems to fluctuate depending on whether or not she (or her husband) is running for office.

Below is a chart of Clinton's net trustworthiness over time -- that is, the percentage saying they consider her to be trustworthy versus the percentage that don't. This is from a slew of polls and using different questions*, so it's not entirely apples-to-apples. And there are apparent outliers -- the blue circles that are some distance from the dark blue line.

In 2008, Clinton's net trustworthiness dropped a ton, and then recovered while she was secretary of state. In 2016, it's dropping again. In the background, you can see the net favorability rating, the thin blue line. It goes up and down depending on if a Clinton is running, dropping in 1996, 2000, 2008 and once again this year.

That doesn't mean that people aren't actually considering her less trustworthy. It simply means that we shouldn't be surprised that they are doing so, based on past trends.

Clearly, Fiorina's polling shows that it's a winning campaign argument, so we should expect to hear it again in the future. And we should also expect the percentage of people calling Clinton trustworthy to continue to drop.

* In a June 2014 poll, ABC News and the Post asked the question two different ways. Half the pool was asked for a yes or no response to, "Do you think Hillary is honest and trustworthy?" The other half was asked for a yes or no response to "She is honest and trustworthy." In the first case, 60 percent of people responded yes. In the second, 53 percent did.