According to most projections, Hillary Clinton is on track for an electoral-college landslide in the coming election, and Democrats have a good chance of gaining control of the Senate and expanding their ranks in the House. But it’s not entirely clear that Americans will be voting for Clinton. During the past year, she’s had lower favorability ratings than any major-party nominee in history, with one important exception: her opponent, Donald Trump. Regardless of the margin of victory, Clinton will have a mandate problem.

There are two aspects to this predicament and to the challenges it poses to Clinton. The first is specific to who she is and how long she’s been in politics, and it won’t be easy to overcome. If she wins, Clinton, who turns sixty-nine today, will be the second-oldest President to take office, less than a year younger than Ronald Reagan was at his first Inauguration. Age is not necessarily the issue—Reagan in 1980 represented change and renewal—but Clinton’s is underscored by her decades on the political stage.

The journalist Jonathan Rauch once noted that politicians whose public careers have lasted more than fourteen years rarely make it to the White House, unless they reach the Vice-Presidency by the fourteen-year mark. Lyndon Johnson, who served twenty-three years in Congress before becoming Vice-President, is the one exception since the turn of the century. Clinton was first elected to the Senate sixteen years ago, and if you count her time as First Lady she’s been in public service for twenty-four years. (If you add her years as First Lady of Arkansas, the total is thirty-eight.)

Her many years in the public sphere have an obvious upside. She will be, arguably, the most experienced person ever to win the Presidency, and certainly the only one to bring a successful and popular former President with her to the White House. But her decades of experience come with a cost: the accumulated grievances of many political opponents and a large percentage of voters who hold unfavorable views about her. These are nearly impossible to change.

On Monday night, the person whom Clinton would succeed tried to explain this phenomenon. “A lot of this just has to do with the fact that she has been in the trenches, in the arena, for thirty years,” Barack Obama said when Jimmy Kimmel asked him why voters don’t trust Clinton. “And when you have been in the public eye that long, and in politics, folks go after you and they’re trying to find a weak spot, and any mistake that you make ends up being magnified, and ginned up, and there are commercials around it, and a whole narrative begins to build. You know that has an impact on people.”

If Clinton wins, her victory will be historically unique—and precarious—in another way. It is rare for a candidate to win the White House after eight years of rule by the same party. It’s only happened once since the Great Depression, when George H. W. Bush succeeded Reagan, in 1988. Maintaining support for the agenda of the party in power for three terms is extremely difficult, as George H. W. Bush learned when Bill Clinton defeated him, in 1992.

The second, perhaps more manageable, threat to Clinton’s mandate is how she’s likely to win this election: by successfully disqualifying her opponent. Polls consistently show that a majority of voters believe that Donald Trump is unfit to be President, which is the central fact of the race and the decisive reason Clinton is likely to win, despite her many problems. Nothing else really matters if one of the candidates doesn’t pass the basic test of fitness for office.

All of this adds up to a serious problem for Clinton in 2017. She will, of course, be the first woman to win the Presidency, a historic milestone that should be cause for some bipartisan celebration. But her honeymoon period is not likely to last. Her Republican opponents will quickly argue that her election was not a referendum on her immigration-reform plan, her infrastructure spending ideas, or her childcare and family-leave proposals, three legislative priorities she is likely to push during her first year in office. The campaign, they will claim, was simply a rejection of Trump. If Republicans control both chambers of Congress, their argument that Clinton lacks a strong mandate will be even more powerful: while Americans may have given her the White House, they also voted for congressional Republicans to serve as a brake on her agenda.

Republicans will point to Clinton’s unusually low favorability ratings to make their case. How could she have a mandate if more voters have an unfavorable opinion of her than a favorable one? In the fall of 2008, some eighty per cent of Obama supporters told pollsters that they were voting for him rather than against his opponent, John McCain. In a new ABC poll released this week, only fifty-six per cent of Clinton's supporters say their vote is for her rather than against Trump.

The good news for Clinton is that this measure of affirmative support has risen nine points since July. Between Election Day and the Inauguration, on January 20th, her popularity is likely to rise further, but as soon as she enters the partisan fray and begins pushing an agenda on Capitol Hill she may find herself stuck with the same middling favorability numbers that she has struggled to overcome throughout the campaign.

There are still two weeks until the election, when views about her mandate will start to freeze. If Clinton wants to make a stronger post-election case for her agenda—and overcome the structural problem of governing after her own party has held the White House for eight years, and after so many years in the political spotlight—then she should shift away from her argument about why Trump is unfit to be President and toward a detailed description of what she will do if voters elect her. If she can lay out in clear terms that her election is about a specific set of proposals that she will offer in her first year, she will be able to make the case later on that her victory was not just a rejection of someone unfit to serve but, rather, an affirmative vote for the Democratic policy agenda. That may be the best way for her to truly make history.