On a conference call with reporters just now, Senator Sheldon Whitehouse made official what I reported here on Friday: This week, he will introduce a bill that would ensure that millionaires paying lower tax rates than middle class taxpayers would henceforth pay a 30 percent tax rate.

Whitehouse clarified that the proposal — the first concrete legislative vehicle for implementing the “Buffett Rule” — will not tamper with existing tax rates. Instead, under the proposal, those making more than $1 million a year would be required to calculate their overall tax rate, taking into account all their income and the full sum of what they pay in taxes. If that amount adds up to less than 30 percent, they would be required to make up the difference.

Whitehouse boiled down the proposal into one line: “If your income is over 1 million, multiply it by 0.3, and if that number is bigger than you’d otherwise be paying, pay that.”

Interestingly, Whitehouse confirmed that the White House had been notified about what he’s up to, which suggests Obama advisers are happy to see this proposal proceeding, at a minimum for the purposes of discussion.

“We’ve reached out to the White House to let them know that we’re doing this,” Whitehouse said.

In his state of the union speech, Obama called for millionaires to be required to pay a tax rate of 30 percent, without offering specifics. Whitehouse’s proposal offers a specific way to make this happen, so it’ll be very interesting to see how it’s received by Obama, the Dem leadership in the Senate, and of course, Congressional Republicans.

Whitehouse said he’d already attracted two cosponsors for the bill and that more would be rolled out in coming days, so keep an eye on what kind of momentum builds behind it and what Senate Dem leaders say about it.

“People believe that the American tax system is not fair, and that the more lobbyists and the more wealth you have, the more goodies you get out of the tax system,” Whitehouse said. “This is a welcome proposal and I hope this will be supported across the board.”

Of course, it won’t be supported across the board; Senate Republicans are all but certain to oppose it. If Dems play their political cards right, they could force a high-profile vote on this proposal — one that Republicans will vote down en masse — even as the GOP appears set to pick a nominee who is worth $250 million and is personally benefitting to an enormous degree from the loophoples the proposal is designed to fix, on behalf of the middle class.