Marco Rubio gives a speech at CPAC, and says that most Americans agree with him. Politifact explains why the statement "a majority of Americans are conservative," which is not technically true, is actually "mostly true."
For 2011, Gallup found that the largest group of Americans identify as conservative, at 40 percent. Another 35 percent identify as moderate, while 21 percent identify as liberal... Rubio said that the majority of Americans are conservative. A respected ongoing poll from Gallup shows that conservatives are the largest ideological group, but they don’t cross the 50 percent threshold. So we rate his statement Mostly True.
We could join the pile-on and explain that there's a perfectly good word to describe the phenomenon of "most but not a majority" -- a "plurality." But I choose to join a different pile-on: The backlash to the Politifact project writ large. The point Rubio was making was that, in elections and moments of trial, it turns out that various things conservatives believe are popular. That's not entirely right, but in 2010, it was. It's not a statement you can apply a fact-check supercomputer to, because it's neither true nor entirely false.