Sen. Marco Rubio has proposed a historically large tax cut. But it's not big enough to spare him from attacks on his right flank.

"It's a welfare transfer payment," Kentucky Sen. Rand Paul dubbed Rubio's tax plan in a presidential debate last month.

Paul, a libertarian rival to Rubio for the GOP nomination, was opining on the Florida Republican's proposed $2,500 per child tax credit. It's a preview of attacks to come as Rubio continues to gain in polls and among Republican donors.

But the tax credit is more than just one campaign plank among many. It represents a toehold in the Republican mainstream for a group of conservatives trying to change the party.

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