He thought that, in America, the wealthiest should pay their fair share, and he said so. I know that position might disqualify him from the Republican primaries these days but what Ronald Reagan was calling for then is the same thing that we're calling for now: a return to basic fairness and responsibility; everybody doing their part. And if it will help convince folks in Congress to make the right choice, we could call it the Reagan Rule instead of the Buffett Rule.
Yet Reagan also championed the very same "trickle down" economics that Obama has roundly denounced--the idea that tax cuts for the wealthy lead to investment that generates growth, and thereby jobs.
Reagan did champion trickle-down, but he did so with provisos that the laissez-faire capitalists decry now: The Laffer Curve. I don't ultimately agree with any such curve because I'm not a capitalist ideologue, period. However, within the current American mixed economy, it really is true that the government can under tax to the detriment of the whole, including the richest, who are routinely shortsighted on these matters. I'm not saying that that was Laffer's point though -- that the government can under tax. Afterall, one could argue whether government "balance" should be achieved where there's supposedly too much socialism. I don't buy the supposition though. Socialism is better than capitalism if the right ones are running it (a fully informed people rather than top-down dictators).