It's a truism among some in the Beltway that the presidential race is already over; that the GOP had best stop supporting Donald Trump* and go about saving the downballot candidates stat.
However, it would certainly be historically unprecedented for any party to do this with such a comparatively close race. It's true that Clinton had led Trump in the RCP average for most of 2016 (aside from a brief blimp after the RNC in Cleveland), which suggests that she has an advantage. However, as of this writing, her advantage is only 2.4 points in a four-way race, and that advantage has been shrinking. That's not an inconsiderable lead, but rarely before has a 2.4-point gap on Labor Day weekend been taken as a sure sign of presidential doom.
According to Gallup records of the 2000 election, George W. Bush led Al Gore by over 10 points as late as October 2000, but the Democratic party didn't immediately throw in the towel on Gore, who ended up winning the popular vote in 2000. After Mitt Romney's convention bounce wore off, President Obama led him by between 3 and 4 points in the RCP average throughout much of September 2012. Yet plenty of observers did not then believe that Governor Romney was fated for defeat.
Conversely, in 1996, the year many 2016-is-over proponents cite, Bill Clinton was absolutely hammering Bob Dole in Gallup polling. Throughout most of the fall of 1996, Dole usually was at least 10 points behind Clinton in Gallup polls. Often he was down between 15 and 20 points. Gallup is not an outlier here; Pew also showed Clinton with a huge lead in the fall. Senator Dole ended up closing the gap in the last week or so of the election (he eventually lost to President Clinton by 9 percent of the popular vote), but polling throughout much of the fall was far more brutal for Dole than it has been for Trump so far. Senator Dole couldn't even break 40 percent in most Gallup and Pew polling.
Now, political dynamics have changed over the past twenty years, and it's possible that a 2-point lead is the new 15-point lead. Donald Trump is also a somewhat unprecedented candidate. And none of this means that Trump will win; the debates and how the third-party vote shakes out in particular could change the trajectory of the race. Nevertheless, in recent political history, no party has given up on its presidential candidate over an almost margin-of-error shortfall in the national polls. There might be other reasons to not support Donald Trump, but the assumption that the presidential race is already over isn't one of them.
*Not by replacing him on the ballot by someone else; instead, he would stay on the top of the ticket and the party would distance itself from him and cut off financial and logistical support.