The bulk of conservative groups are rallying behind the House Republican tax bill.

Forty free-market groups sent letters to all U.S. representatives Thursday, telling them that the Tax Cuts and Jobs Act's "provisions are a victory for the middle class, providing new tax relief, enabling more families to save and invest for the future, and creating an economy more conducive to prosperity for middle-class workers."

The letter was led by the American Action Network, an outside group aligned with Republican leadership, and signed by Freedomworks, Heritage Action, the National Taxpayers Union, and more.

Some right-of-center groups have flagged concerns with the bill. The Club for Growth, for instance, called one provision an example of "class warfare."

Americans for Prosperity, the Koch brothers-affiliated group that has spent millions backing action on taxes, also has raised worries about the bill's treatment of foreign business earnings. A representative for the group said that it still had some concerns about the measure, but was hopeful for changes to the bill before it passed.

Nevertheless, many of the outside groups are on board.

"We're sending a clear message of support to members of Congress," said Corry Bliss, executive director of the American Action Network.