With crowds of supporters waiting for her, Secretary Hillary Clinton stopped at a coffee house in Mount Vernon, Iowa, on April 14, 2015. (Melina Mara/The Washington Post)

Is Hillary Clinton honest enough to be president?

That question — phrased in a thousand different ways but always with the same doubts in mind — sits at the heart of a campaign that will span the next 18 months and on which billions of dollars will be spent.

It speaks to the seemingly contradictory reality of Clinton as a candidate: She is her own best asset. She is also her own worst enemy.

There is little doubt among the electorate — with the exception of conservative Republicans who will never vote for Clinton under any circumstances — that her life experiences and résumé have prepared her to do the job. First lady, senator from New York, secretary of state — no one in the field (on either side) can match those credentials.

Clinton is universally known and, generally, regarded as hyper-competent. That’s her as her best asset.

Former U.S. senator and secretary of state Hillary Clinton announced that she’s running for president in 2016. Here's the Democrat’s take on women’s rights, Benghazi and more, in her own words. (Julie Percha/The Washington Post)

Then there is the other side of being Hillary Clinton, the not-so-good side if you are running to be the next president. Yes, she has rolled up an unmatched résumé over the past two-plus decades in national public life. But, in that same period, she and her husband have been dogged by a general perception that they don’t always play by the rules and, in the most negative reading, don’t think the rules apply to them.

From Whitewater to Travelgate to Monica Lewinsky to the Marc Rich pardon (and beyond), the Clintons’ time in and around Washington has been defined by questions of propriety — of bending (some would say breaking) the rules to benefit them and their friends.

And it has left a mark.

A new Associated Press-GfK poll shows that 6 in 10 people, or 61 percent, said “honest” describes Clinton only slightly well or not at all. Before you argue that those numbers are just recalcitrant Republicans corrupting the sample, chew on these two nuggets: First, 4 in 10 Democrats — yes, Democrats — said “honest” either barely applied to Clinton or didn’t apply at all, and, second, 6 in 10 independents said the same about Clinton’s “honesty” problem.

Those are remarkable numbers for a candidate not expected to face serious opposition for the Democratic presidential nomination and who polling suggests will start as a favorite in the general election next year against any Republican candidate.

The Clinton conundrum is perhaps best explained by numbers found within a Quinnipiac University poll conducted last month.

More than 6 in 10 voters (62 percent) said that Clinton has “strong leadership qualities.” In that same sample, though, less than 4 in 10 (38 percent) said she was honest and trustworthy. A majority (54 percent) said she is not honest and trustworthy, including 61 percent of independents.

Those findings seem contradictory, right? One way of reconciling them is to see Clinton’s high numbers on leadership as a judgment on her professional life, and her low numbers on trust as a reflection of people’s dislike of how she and her husband have conducted their private affairs.

Of course, the split between public and private in politics is becoming increasingly blurry, and by the time the 2016 campaign heats up, it may not exist — if it even does now.

And, Clinton is not helping her cause. A recent spate of stories raise questions about the possibility of a too-close relationship between the former first family and big donors to the Clinton Foundation.

Republicans are seizing on those stories to stoke doubts about whether Clinton is honest enough to be president. “It’s the Clinton way: raking in millions from foreign governments behind closed doors while making promises about transparency that they never intended to keep,” said Carly Fiorina, a former Hewlett-Packard chief executive who is expected to formally declare her bid for the Republican presidential nomination Monday. “Have we had enough of a ruling political class that doles out favors to the wealthy and well-connected few?”

That line of attack is hugely problematic for Clinton’s presidential bid because it speaks to the concerns about trust that many in the electorate — including many who are favorably inclined to the Clintons — feel toward her and her family.

It affirms for people that there is always some piece — or pieces — of baggage that comes with electing the Clintons to anything. It’s part of the deal. You don’t get one without the other.

The question at the heart of the election, then, is whether Clinton’s competence or readiness to be president outweighs doubts about whether she can be fully trusted with the office for which she is running.

Answer that, and I’ll tell you whether Clinton wins in 2016.