The current version of the tax plan features a 21 percent corporate rate, two people briefed on the plan told CNBC.

It also would feature a top individual tax rate of 37 percent and allow homeowners to deduct mortgage interest on loans up to $750,000, up from the $500,000 cap in the House version of the bill. This version would also lower the pass-through deduction back down to 20 percent.

One source said that this corporate rate could take effect in 2018. But a White House official said these details were part of the "current state of play," as negotiations continue.

The official said that both House and Senate members of the conference committee are expected at the White House for lunch on Wednesday.

Republicans are rushing to hash out a joint tax bill that can pass both chambers of Congress before Christmas. A conference committee is trying to work out the differences between the separate bills passed in the House and Senate.

— Reporting by CNBC's Ylan Mui and Eamon Javers.