Thurday will be an interesting test of the ability of Congress to crack a Bush administration coverup of a rotten and likely illegal action: its decision to reject California’s effort to enforce its greenhouse-gas standards for motor vehicles.

Sen. Barbara Boxer will put EPA Administrator Steve Johnson in the box to explore not only his indefensible decision, but his efforts to withhold information from Congress and cover up the truth about his pro-car company action.

You will recall that right before Christmas, Johnson nixed the California request in a hastily called news conference where he tried, dishonestly, to spin his way out of a looming Washington Post exclusive.

The lies continued last week as the EPA — on the Friday of a holiday weekend, in an effort to minimize attention — sent Boxer a letter and portions of various materials she sought. Boxer noted much of the relevant information was “whited out,” as EPA Associate Administrator Christopher Bliley literally invoked the Nixon Watergate coverup as justification.

Contrary to initial reports, the EPA stopped short of invoking executive privilege. (That only pertains to materials that go to the president himself, or his top aides. If the administration ultimately does invoke that, it would be an admission that the White House was directly involved in this — something Johnson has denied.)

Even so, the EPA letter was something that could be submitted to a fiction-writing contest. Bliley asserted that revealing what EPA experts told him (namely, that California deserved to receive its waiver and that EPA would probably lose when California sued) would produce a “chilling effect” on EPA staff morale. (Johnson himself froze them out of the decision.) Bliley also claimed that the public would be confused by the release of “pre-decisional” documents (in other words, what EPA staff wrote before the political censors went to work).

Boxer isn’t the only one in Congress digging into the coverup. Rep. Henry Waxman, chairman of the House Committee on Oversight and Government Reform, wants to depose key EPA officials (PDF).