I'm guessing it will be about 54%.
It's going to depend on the
direction of the shift.
It's hard for the D+3.85 margin, by which Barack Obama won the U.S. Popular Vote for his 2012 re-election, to be expected to shift Republican without the Republican Party and their nominee also winning over the U.S. Popular Vote (and, along with that, the Electoral College).
It's likely, if the Democrats win a third consecutive cycle (with or without the party nod going to Hillary Clinton), the popular-vote margin would become an increase.
One way you can tell this is with my favorite of all demographics: the gender votes. But cross-reference to all of one's delight and, aside from Hispanics and African-Americans and Asians and Others, one may want to ask if he thinks the Republican nominee (if that party loses a third straight election) will reach Mitt Romney's 59 percent of the White vote nationwide. (A 20-point margin of national carriage of Whites.)
As usual, much answering is the same: it depends on the national
conditions. But with realigning presidential periods, it has not yet happened (when the in-party won seven of nine or ten election cycles) for the out-party to stop the in-party from having at least one occurrence of winning beyond two consecutive presidential election cycles. (From 1860 to 1892, Republicans won six in a row. From 1896 to 1928, Republicans won four in a row and, in the 1920s, three in a row. From 1932 to 1948, Democrats won all five cycles of the 1930s and 1940s. From 1968 to 2004, Republicans won all three from the 1980s.)