Sylar told claire that he met a special girl who allowed him to become president. I took this to mean Candice which would allow him to be Nathan - the president.
Also in Isaac's loft Sylar/Nathan said "...I know how things work!"
I had to watch the episode twice to take everything in. The clues are there if you know to look for them.
surely if Isaac had painted Sylar as Nathan in the whitehouse himself, it'd have looked more in line with what sylar painted.