Four huge automakers — Ford, Honda, VW and BMW — said Thursday that they've struck a voluntary deal with California on vehicle emissions rules that they're hoping will form the basis for a nationwide agreement with the Trump administration.

Why it matters: It signals a break between the companies and the White House as the Trump administration pushes ahead with plans to weaken mileage and emissions standards.

The Trump administration is planning to freeze annual emissions standards increases in 2020, rather than letting them grow stricter as envisioned under Obama-era rules.

The EPA, as part of that plan, would seek to strip California's special permission under the Clean Air Act to set its own emissions standards that a number of other states follow.

Yes, but: While major automakers had backed efforts to weaken Obama-era rules they call too aggressive, they have also bristled at Trump's push to freeze them outright.

The big picture: According to Reuters, under the deal with California, "the stringency of the requirements would increase at a nationwide average annual rate of 3.7% annually starting in the 2022 model year through 2026, and 1% of that annual improvement could be covered by credits awarded for building electrified vehicles."

What they're saying: "These terms will provide our companies much-needed regulatory certainty by allowing us to meet both federal and state requirements with a single national fleet, avoiding a patchwork of regulations while continuing to ensure meaningful greenhouse gas emissions reductions," the four automakers said in a joint statement.

They added that the agreement is "available to all automakers."

Mary Nichols, California's top air quality official, told the Washington Post that she sees the deal as an "olive branch" to the Trump administration.

Go deeper: How California can meet its aggressive clean energy law