The next day, Sen. Tom Cotton took to the Senate floor to slam Ryan’s so-called border adjustment tax, saying “some ideas are so stupid only an intellectual could believe them.”

“Many other senators share these concerns and we most certainly will not ‘keep our powder dry,’” Cotton went on, without naming the speaker in his speech.

The sequence was an ominous sign for a linchpin of Ryan’s tax plan — and perhaps for the prospects of tax reform happening at all. The border adjustment tax would generate more than a trillion dollars over a decade; there’s no obvious way to replace that money, which is needed to help pay for a steep cut in corporate and income taxes.

In meetings with administration officials and Senate leaders, Ryan has framed his proposal as a compromise between a tariff, which the president wants, and conservative orthodoxy against border taxes. He has suggested it’s in keeping with President Donald Trump’s “America first” mantra, since it would reward American manufacturers that make products here and sell it abroad.