I guess that all depends on whose definition of "conservative" you want to use.
If you mean the definition of conservative which embraces protectionism, isolationism, and non-interventionism as a political ideology, then they are not...in fact, they have never truly been the base because all those are political ideologies which have never been embraced by the U.S. people at any level.
If you mean the "conservatives" who support exclusion rather than inclusion, who see dialog with certain segments of the voters as "pandering", and who raise ideology over people, they haven't been any part of "the base" since Reagan raised the standard, and placed people ahead of ideology.
And lastly, if you define "conservatives" as they who would welcome electoral victories by Democrats in the name of advancing "conservative" principles, then I say to you that they are truly NOT the base of the Party.
The base of anything must, by definition, be that which supports and stabilizes it, the "conservatives" around these parts, or at least those who label themselves "true conservatives" have left the Party before, and threaten to leave it again...they cannot possibly be the base.
Like it or not, they are the minority within the majority membership, and geometrically speaking, if you see the ideological make up of the GOP as a triangle, these "conservatives" are the point, and the narrowest point of a triangle can't be its base.