Actually, there aren't that many. What are the odds that the incumbent party will hold onto the White House when its deeply unpopular incumbent isn't on the ticket? The two main precedents that jump to mind are 1952 and 1968. In both examples, however, the president declined to run for an additional term, largely because he realized he would have lost. (Bush isn't running next year because he's termed out.) But both examples include presidents who served more than one term (Truman took over when FDR died; LBJ took over when JFK was shot) and had started - and failed to end - unpopular wars. And even though the incumbents weren't on the ticket, their party still lost the presidential election. Possibly, 1920 could fall into this category. WWI was over, of course, and we don't have reliable numbers on Wilson's popularity, but it seems safe to assume that the League of Nations fight and his stroke took a toll on his approval ratings. And his party lost in the election, as well.
The Republicans will, of course, be in this same situation next November. I'm guessing the Republican nominee will eventually try to distance himself from the war, but none of the frontrunners have remotely done that yet. Quite the contrary. So the Republicans will be at a real disadvantage. That doesn't guarantee a Democratic victory. Remember that the Republican victories in 1952 and 1968 were hardly blowouts. But the wind is against the Republicans.