House Oversight and Government Reform Committee Chairman Darrell Issa (R-Calif.) faces off with ranking member Elijah Cummings (D-Md.) after former Internal Revenue Service official Lois Lerner once again refused to answer questions at a congressional hearing. (The Washington Post)

A long-simmering battle over whether the Internal Revenue Service targeted conservative groups for extra scrutiny with political intentions has heated up again on Capitol Hill.

A House hearing erupted into shouts of frustration and recrimination Wednesday morning in a scene reminiscent of the early days of the probe into the IRS activities.

There again was Lois Lerner, the former IRS official who last year invoked her Fifth Amendment right against self-incrimination, declining once more to testify before the House Oversight and Government Reform Committee.

On Wednesday, Lerner again invoked the Fifth Amendment, ending days of speculation over whether she would testify after being subpoenaed.

The shouting began as a clearly frustrated committee chairman Darrell Issa (R-Calif.) adjourned the hearing and the equally annoyed ranking Democrat, Rep. Elijah Cummings (Md.), objected.

“Mr. Chairman, you cannot run a committee like this,” Cummings said.

After his microphone was cut off, Cummings angrily shouted: “I am a member of the Congress of the United States of America. I am tired of this.”

He also described Issa’s investigation into the IRS matter as “one-sided” and “absolutely un-American.”

“We had a hearing,” Issa replied. “It was adjourned. I gave you an opportunity to ask your questions. You had no questions.”

And so it goes, a fight that for some observers feels never-ending, and makes the political battles over an alleged Benghazi coverup look tame by comparison.

After lawmakers spent months last year lobbing charges and countercharges, the IRS controversy appeared to quiet down.

But what, on some days, had seemed more like a political food fight than a focused bipartisan probe is blowing up again, this time with two pivotal factors looming. In addition to the approaching midterm elections, there is the controversy over a Treasury Department proposal to restrict the political activities of nonprofit groups with 501 (c)(4) tax status, or so-called social welfare organizations.

Count on both of those dynamics — not to mention any valid evidence that emerges on either side of the debate — to keep the IRS issue front and center.

Social-welfare organizations have pumped hundreds of millions of dollars into election-related ads since the Supreme Court’s 2010 Citizens United decision, which said the government cannot restrict independent political expenditures from certain types of groups.

The IRS has said its problems partly stemmed from being overwhelmed by applications from such groups and a lack of clarity on how to deal with them.

Republicans, though they have not proven a political connection to the White House, believe the IRS scrutiny was an attempt by the Obama administration to silence conservative critics.

Democrats have said the problems were nothing more than hapless bureaucratic fumbling and missteps. They have also uncovered evidence that progressive groups were targeted for intense scrutiny.

Now the Treasury’s proposed rules have caused a firestorm of their own.

The draft guidelines would define a set of “campaign-related political activities” that could disqualify groups from tax-exempt status, but the draft rules place voter-registration and get-out-the-vote efforts into that category.

Those rules represent one of the few areas of common ground in this episode: Conservatives and liberals alike have said the plan goes too far, arguing that it could hinder free speech. The GOP-controlled House last week passed legislation that would block the proposed guidelines from taking effect.

Democrats are more intent on tweaking the planned changes than scrapping them. Last week, hours before a midnight deadline for public comment on the draft proposal — there were more than a 122,000 submissions at the time — a dozen Democratic senators recommended limiting the political activities of nonprofits to between 5 and 15 percent of their overall efforts.

IRS guidelines say the groups must be “primarily” involved in social welfare, suggesting they can participate in some level of electioneering.

The lawmakers also said in their letter that the definition of political activity should exclude voter registration and get-out-the-vote drives, but they argued that the free-speech concerns lack merit, saying “these rules would not restrict anyone’s right to speak, or to spend money to influence elections.”

The scandal has its roots in a report from Treasury inspector general J. Russell George that said the IRS had targeted nonprofit advocacy groups.

Lerner had foreshadowed the findings days before the report’s release when she apologized at a legal conference for the agency’s “absolutely inappropriate” actions toward groups with terms such as “tea party” and “patriot” in their names.

Republicans pounced, suggesting the IRS actions were part of an administration plot to silence President Obama’s critics. Their investigations through at least three committees have focused largely on proving their suspicions. Issa’s panel alone has held five hearings and interviewed nearly three dozen IRS officials.

The IRS has said its handed over to Congress more than 500,000 pages of documents.

Democrats contend that their GOP colleagues are pushing a false conspiracy theory based on cherry-picked facts. They stepped up their counter-campaign last month, questioning George’s independence.

Reps. Gerald E. Connolly (D-Va.) and Matthew Cartwright (D-Pa.) have asked a special watchdog to investigate whether George handled the probe fairly, suggesting the review focused too much on an IRS “be on the lookout” list that targeted exclusively right-leaning groups, while ignoring older lists that contained some terms associated with progressives.

George defended his audit in an interview, telling The Washington Post, “We noted there were other ‘be on the lookout’ lists that included other types of organizations, but that was not the initial charge of the review and not the focus.”

Republicans have shifted the focus back to the administration, questioning its placement of an Obama campaign donor help lead a Justice Department investigation into the targeting matter.

The inspector general’s review and the Obama administration may both deserve further examination, but lawmakers, Republicans and Democrats alike, have muddied the investigations with efforts to score political points.

Where it will all end is unknown.

After Lerner invoked her Fifth Amendment right Wednesday, House Speaker John A. Boehner (R-Ohio) said she should be held in contempt — a move that would intensify the controversy.