That standard, however, has not kept Clinton mum on other issues. Her State Department worked on striking a nuclear deal with Iran, combating terrorism and pursuing trade deals with the United States' Asian allies, all issues that Clinton has commented on as a 2016 presidential candidate.
Her criticism of the Trans-Pacific Partnership has echoed some of the complaints firebrand liberal Sen. Elizabeth Warren and manufacturing-state populists like Sen. Sherrod Brown have lodged, giving cover to Democrats whose allegiances are divided by the deal.
"There are some specifics in there that could and should be changed," Clinton said of the Pacific Rim pact last month in Iowa. "So I am hoping that's what happens now -- let's take the lemons and turn it into lemonade."
After meeting with top officials from the AFL-CIO union on Thursday, Clinton explained why she feels she can comment on trade but not Keystone: She never worked directly on the Trans-Pacific Partnership.
"I did not work on TPP," Clinton said Thursday. "I advocated for a multi-national trade agreement that would 'be the gold standard.' But that was the responsibility of the United States trade representative."
Clinton added, "I never had any direct responsibility for the negotiations at all. I certainly expressed the opinion that if we could get the right kind of agreement that would protect American workers, raise incomes, and increase growth, and be in the interest of our national security, that would be a positive for America. But it was not my responsibility to negotiate it or to run the process that was negotiated, unlike the Keystone example which you raised."
Clinton is right: Her State Department never directly negotiated the trade the massive 12-nation trade deal that is a staple of the Obama administration's foreign policy in the region. That responsibility was left to the United States Trade Representative.
But she did publicly and actively advocate for the TPP. In fact, she did so 45 times between 2010 and 2013
During a 2012 trip to Australia, for example, Clinton urged countries in the region to work specifically on the Trans-Pacific Partnership.
"We need to keep upping our game both bilaterally and with partners across the region through agreements like the Trans-Pacific Partnership or TPP," she said. "This TPP sets the gold standard in trade agreements to open free, transparent, fair trade, the kind of environment that has the rule of law and a level playing field. And when negotiated, this agreement will cover 40 percent of the world's total trade and build in strong protections for workers and the environment."
And during a 2010 meeting in New Zealand, Clinton said she was "very committed to the Trans-Pacific Partnership."
"We want to expedite the negotiations as much as possible," Clinton said. "So we are exploring ways that we can try to drive this agenda. I am absolutely convinced that opening up markets in Asia amongst all of us and doing so in a way that creates win-win situations so that people feel that trade is in their interests."
She has also advocated in recent weeks for the Iran nuclear deal -- which Clinton's back channel talks helped set into motion and which her campaign's policy adviser, Jake Sullivan, began negotiating as a top aide Clinton's State Department. Clinton has thrown her support behind the controversial pact that the United States and other world powers struck with Iran, drawing the ire of the entire Republican presidential field.
But Clinton refuses to comment on Keystone.
During campaign swings through Iowa and New Hampshire in the last week, Clinton declined to take a position on the pipeline, a 1,179-mile-long project that would move oil from Canada to refineries in the United States but has drawn the ire of environmentalists.
"I am not going to second guess (Obama) because I was in a position to set this in motion," Clinton said in New Hampshire
, referencing environmental reviews conducted by the State Department that began when she was secretary of state. "I want to wait and see what he and Secretary Kerry decide."
She added, "If it is undecided when I become president, I will answer your question."