WASHINGTON — President Obama has made the Buffett Rule, mandating that millionaires pay at least 30 percent of their incomes in taxes, the centerpiece of his campaign for “fairness.” But look for it among the myriad tax changes the White House detailed in the 2013 budget proposal it released this week, and you will not find it.

The Buffett Rule has become a signal piece of election-year political rhetoric. It featured heavily in Mr. Obama’s State of the Union address last month. It has become a favored talking point on the campaign trail. And Mr. Obama underscored his support of it in his budget proposal.

But the White House says it is a “guideline,” rather than a legislative initiative. And it says it prefers not to establish the Buffett Rule without a broader overhaul of the tax code, though it would support a Congressional effort to carry it out alone.

“This is the guiding principle of tax reform,” said Jason Furman, principal deputy director of the White House’s National Economic Council. “To some degree, it’s a specific policy, where we set a floor, a minimum rate. And to some degree, it is a statement of principle of how you would like to design the tax system.”