I think it would slow down slightly as by that time you will start seeing more baby boomers die off. Its possible in 2032 (though that might be a stretch) or 2036, but it would take either a landslide or a very high turnout election (such as 04 to achieve that). More than likely it would be 2044 at the earliest and possibly not till 2052.

Around 2050 is probably most likely. A total of 122 Mio. people voted in 2004, 69 Mio. in 1960. Therefore the total number of votes increased by 1,3% each year - along population growth during that time.

Let´s assume a declining 1% growth until the 2052 election, which brings the total number of voters to 197 Mio. in that year.

A candidate in 2052 then has to win with 51% of the vote to get more than 100 Mio. votes ...