Sen. Rand PaulRandal (Rand) Howard PaulTrump formally sends Pompeo nomination to Senate Spending bill delay raises risk of partial government shutdown support Overnight Defense: Senate sides with Trump on military role in Yemen | Dem vets push for new war authorization on Iraq anniversary | General says time isn't 'right' for space corps MORE (R-Ky.) is wasting no time going after Jeb Bush, his likely rival for the Republican presidential nomination in 2016.

Just hours after Bush announced on Tuesday that he was “actively exploring” a presidential bid, Paul went on Fox News and predicted the former Florida governor’s support for Common Core education reform would be a big problem for him in the Republican primaries.

“For Jeb Bush to run in the primary will be very, very difficult because if you’re going to be for a national curriculum and for Common Core and for No Child Left Behind, this accumulation of power in Washington, that’s not very popular and it’s going to be overcoming if he thinks he can win the primary,” Paul said on "The Kelly File."

Common Core, a set of nationalized education standards that has been adopted by 43 states, is unpopular among many in the GOP.

“I think it will be very difficult [for Bush], most of us believe in less federal government and more decentralized government, particularly with education,” Paul said Tuesday. “You know, Ronald Reagan ran on the platform of getting rid of the Department of Education. We’ve always believed in decentralized education.”

Bush has made education reform the centerpiece of his work as governor of Florida and since leaving office, and some of his views on education are a rallying point for Republicans. Bush is a strong advocate for charter schools and school voucher programs, and he counts Speaker John Boehner (R-Ohio) as a strong ally on those matters.

Education is low on most voter's list of concerns, but critics of Bush have cited his support for Common Core and his stance on immigration reform as evidence he’ll have a hard time rallying the conservative voters who turn out for primaries and caucuses in early-voting states.

Still, Bush is near the top and leading Paul in most polls of Republican voters right now.

Paul has said he’ll decide whether he’ll run for president in early or mid-2015. He speculated that Bush was making moves so early in the process because he’s been out of office for so long and his political skills are rusty.

“Maybe he has more ground he needs to gain,” Paul said. “He’s been out of this a while so maybe he needs to get back in and practice up a bit.”