The best-known forecasting tool of the bunch—and one that plainly spells out Clinton's looming trouble—is Abramowitz's "Time for Change" model. He first built it before George H.W. Bush's 1988 election, and he has used it to predict the winner of the popular vote in the seven White House races since. (The model predicted that Al Gore would win the presidency in 2000, when he became the first person since Grover Cleveland to earn the majority of the popular vote nationally but lose the Electoral College.)
The model uses just three variables to determine the winner: the incumbent's approval rating, economic growth in the second quarter of the election year, and the number of terms the candidate's party has held the White House. Official forecasts aren't made until the summer before the presidential election. But reasonable estimates rooted in current political and economic conditions demonstrate Clinton's vulnerability.
Consider this scenario: President Obama retains equal levels of approval and disapproval, better than he has had most of his second term; and gross domestic product growth in the second quarter of 2016 holds at 2.4 percent, the same as last year's rate of growth. Under this scenario, the "Time for Change" model projects that Clinton will secure just 48.7 percent of the popular vote.
In other words, she loses.
Slight increases in Obama's approval rating and economic growth aren't enough to change the outcome for Clinton. Every 10-point improvement in the president's approval—if, for example, 55 percent of voters approved of Obama while 45 percent didn't—earns Clinton only an additional 1 percentage point of the popular vote. It takes an extra 1 percent year-over-year GDP growth to give Clinton an extra half percentage point of the popular vote.
For Clinton to reach 50 percent of the popular vote, under this model's rules, the president would need to see a 5-point increase in his approval rating and GDP growth would have to hit 3.5 percent. It's certainly possible, but it's fair to call that a best-case scenario for Obama in his final year as president.