nlike most of the other Republican 2016 hopefuls, Jeb Bush has pointedly refused to rule out raising taxes on the wealthy, opening up another divide between the ex-Florida governor and conservatives in his party.

Americans for Tax Reform, an influential conservative group, has for more than two decades circulated a pledge in which candidates commit to opposing any net tax increase. Most Republican members of Congress have signed it, as have more than a dozen GOP governors and about 1000 state legislators, according to ATR.

Sen. Rand Paul of Kentucky, Sen. Marco Rubio of Florida and Sen. Ted Cruz of Texas have signed the pledge as presidential candidates over the last few weeks. They have committed, in the words of the agreement, to oppose "any and all efforts to increase the marginal income tax rate for individuals and business" and "any net reduction or elimination of deductions and credits, unless matched dollar for dollar by further reducing tax rates."

Aides to Wisconsin Gov. Scott Walker say the governor is likely to sign the pledge when he becomes an official candidate.

"You're asking for the votes of millions of people. 'Trust me' does not work on matters that concern other people."

But Bush's aides say the ex-governor won't sign any pledges during the 2016 campaign cycle, including the ATR one. And when pressed to rule out tax increases at the recent National Review Ideas Summit in Washington, Bush repeatedly evaded the question, emphasizing his record of cutting taxes in Florida but refusing any commitments.