Using some very rough math, it seems about 23% of the entire U.S. population voted Obama in 2008. A landslide on that level with 100 million voters for the winner would require a population of something like 435 million Americans. Most projections indicate that if turnout continues to be under 60%, this shouldn't be possible until after 2050. Since we're not exactly having a baby boom right now, nor are the conditions there to have one in the future, I'd say it becomes possible starting in 2068. I'm not a math guy, so someone can probably do this better than me.
Of course, if there are some New Deal-style sweeps by a dominant Democratic Party in the next few decades, it could be much sooner. But we're talking about maps that only a hack would make right now, and with high turnout to boot.
You called me (lol not really)
Crunching the numbers it seems that Nixon in 1972 and Reagan in 1984 also took the votes of about 23% of the entire U.S. population despite winning much bigger landslides with not too dissimilar turnout, which is I suppose a product of the U.S. population being now older (so less underage kids).
With the population continuing to age, if turnout remains in the same ballpark, I can see someone having a great victory and taking the votes of 25% of the total population. If turnout (hopefully) swells, I can see the figure reaching 27%.
So we are talking of between 375 and 400 million people, which I suppose means the 2040s or 2050s.