Congressional Democrats have sent a letter to House Republicans formally demanding that they call the author of the now-infamous audit on IRS targeting of conservative groups to come back to the Hill and testify under oath — where he’ll be pressed to explain why the audit failed to detail that progressive groups had also been targeted.

The letter signed by every Democrat on the House Ways and Means Committee — which was sent over by a source — ratchets up the stakes in the battle over the direction of the probe into IRS targeting, at a time when news outlets have cast doubt on claims that the targeting of conservative groups was politically motivated or had any ties to the White House.

The letter — which was addressed to GOP Rep. Dave Camp, the chairman of Ways and Means — is designed to put pressure on Republicans to allow the author of the audit, Treasury Inspector General Russell George, to submit to more direct questioning from Dems in light of new revelations about the “Be On the Lookout” document, or BOLO, that indicates progressive groups seeking tax exempt status were also examined by the IRS. It reads:

Dear Chairman Camp:

It is now evident that the Treasury Inspector General for Tax Administration’s May 14, 2012 audit report titled “Inappropriate Criteria Were Used to Identify Tax-Exempt Applications for Review” is fundamentally flawed.

— The audit fails to acknowledge that the term “progressives” was on many of the same screening lists that included the term “Tea Party.”

— The audit also fails to acknowledge that liberal groups were among the 298 applications reviewed.

We request that you immediately ask Mr. George to return to the Committee and answer questions under oath regarding all of these matters.

This comes as George sent a letter of his own to Congressional Democrats today pushing back on criticism of his audit. His letter — a response to questions about why he didn’t mention progressive groups in the audit — claims that six applications within the period in question had the words “progress” or “progressive” in their names out of 298 political cases identified by the IG as having been scrutinized by the IRS. It also said that while 30 percent of the groups with “progress” or “progressive” in the names were “processed as potential political cases,” the audit found that 100 percent of names with “Tea Party, Patriots, or 9/12 in their names” were processed as political.

But as the Hill put it: “George’s letter does not say why the progressive groups were given extra scrutiny.”

Conservatives have jumped on George’s letter as evidence that the targeting of conservative groups far outweighed that of liberal groups, suggesting political motivation to the targeting.

But if George returns to the Hill, Dems will likely point out that his latest letter does, in fact, acknowledge that progressive groups were designated on the BOLO, and ask him why that was not mentioned in the initial audit. They will also ask him why, when previously asked at a hearing whether some of the groups were progressive, he claimed that “we were unable to make that determination.”

Dems are also likely to press George on the fact that an Inspector General spokesman previously told reporters that progressive groups were not mentioned in the audit specifically because Darrell Issa’s committee asked for a focus only on the targeting of conservatives. This claim was seemingly contradicted in George’s new letter, which says “we did not limit our audit to allegations related to the Tea Party.” Dems will ask for an explanation for the discrepancy.

Dems will likely also ask George to account for his claim that only six progressive groups were scrutinized. They will ask him whether other political groups with liberal leanings that did not have “progress” or “progressive” in their names were targeted, and how many of these groups are in the category of 202 groups vaguely marked “other” that the original IG audit says were originally signaled out as political.

Greg Sargent writes The Plum Line blog, a reported opinion blog with a liberal slant -- what you might call “opinionated reporting” from the left.