WASHINGTON — Lawmakers are on the verge of striking a tentative agreement that would reconcile tax bills passed separately by the House and Senate, and an announcement could come later Tuesday or sometime Wednesday, a top Senate Republican said Tuesday.

Republican Sen. John Thune, R-S.D., the No. 3 Senate Republican, said in an interview that negotiators are reviewing a proposal they received late Monday from House leaders. But he said there is still “swirl” around key details — including the corporate tax rate.

Both bills included as their central feature a massive reduction in the current 35 percent tax rate down to 20 percent. But lawmakers have been discussing pushing it back up as high as 22 percent to free up money for other changes they want to make.

Thune is among the group of lawmakers working to meld the $1.5 trillion bills passed separately by the House and Senate into a final product. Republicans will then aim to push that capstone bill through both chambers of Congress and onto President Donald Trump’s desk before Christmas.

Ever since the deliberations regarding slightly raising the corporate rate became public, conservative groups have been agitating against raising it over 20 percent. So negotiators face significant crosswinds on the issue, illustrating the pressure they are under as they rush to wrap up a final product.

Earlier, speaking to reporters Monday evening, House Ways and Means Chairman Kevin Brady, R-Texas, said numerous details remained in play. One of those is the estate tax, which the House bill repeals fully while the Senate bill only limits. House conservatives have been pushing to keep full repeal, but Brady indicated openness to the Senate approach.

“In the House we feel very strongly about fully repealing the estate tax,” Brady said. “We’re having those discussions with the Senate that took a different approach. They did double the exemption so that helps a lot of family-owned farms and businesses. So that’ll be part of the final agreement.”

Overall, Brady said, “nothing’s final until everything’s final and tax reform is truly a Rubik’s cube.”

“We continue to stay on schedule for a conference committee report and a vote next week all with the goal of getting this to the president’s desk soon thereafter,” Brady said.

Beyond the estate tax, the House and Senate bills have a number of differences that must be resolved.

For example, the Senate bill retained seven income tax brackets for individuals and families, while the House bill collapsed those brackets down to four.

The House and Senate bills also tax partnerships and sole proprietorships much differently. The House bill would also put new limits on how much mortgage interest a household can deduct from their income, while the Senate bill would not make changes.

And the House bill would eliminate the alternative-minimum tax, while the Senate bill would not.

Congressional leaders have already signaled how they plan to resolve some differences between the two bills. For example, the Senate bill would repeal the individual mandate of the Affordable Care Act, while the House bill would not. But House leaders have suggested they like that change to the health care law and support including it in the final bill.

GOP leaders do not see any of the differences as dealbreakers, but they are trying to work fast to reach a resolution so Trump can sign the package into law by Christmas.

GOP negotiators have spent the past week looking at ways to allow people to continue deducting some of their state income taxes from their federal income. This provision wasn’t in either the House or Senate bills, but some California and New York Republicans have insisted that it be included. Such a change could make the bill.

Congressional leaders have set up a “conference” committee to work through differences between the House and Senate bills, and that group of lawmakers is scheduled to meet Wednesday at 2 p.m. These meetings can be long, drawn out affairs with many amendments and all-night debates, but GOP leaders are hoping to have all of their decisions nailed down before the gathering.

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