"The Presidential nominee of each major party may become eligible for a public grant..."
"The amount of public funding to which a minor party candidate is entitled is based on the ratio of the party's popular vote in the preceding Presidential election..."
"Each major political party is entitled to $4 million (plus cost-of-living adjustments)8 to finance its national Presidential nominating convention. A qualified minor party may become eligible for partial convention funding..."
As for the constitutional basis, I agree that there is none. Then again, there is little to no constitutional basis for social security, medicare, the war on drug users, the department of education, etc.
I'm just saying that gop.gov isn't anything special or unprecedented.