Many Democrats were muted in their response to the subpoena, but Cummings reacted quickly. Within minutes, he issued a statement in which he recounted Issa's objections to a Democratic attempt in 2007 to subpoena then-Secretary of State Condoleezza Rice over comments President George W. Bush had made about Iraqi leader Saddam Hussein seeking uranium from Niger.

A transcript of a hearing from that time shows Issa noting Rice's busy schedule, including meetings with the Syrian foreign minister and NATO.

"The inappropriateness of hauling the secretary of state ... out of the performance of her job is what we're objecting to," he said.

Democrats have praised Cummings' work as ranking member on the oversight committee. Rep. Gerry Connolly, a Virginia Democrat and prominent voice on the panel, said Issa had long signaled he would be a "trench warfare guy."

"Elijah had to be willing to go into hand-to-hand combat with Issa, and he has certainly shown he's quite capable of doing that while still calling on us to elevate the conversation," Connolly said.

"I think he's emerged from this combat with far greater stature," he said.

House Minority Leader Nancy Pelosi said in a statement that Cummings is "admired for his courage, commitment to the truth and his compassion for all Americans."

But in early April, Issa released a series of emails from 2012 showing that IRS staff were attempting to respond to a request for information from Cummings' aides about the tea party-affiliated group True the Vote. Republicans suggested that Cummings, by making the request, had inappropriately pressured the IRS to single out the Houston-based group for scrutiny.

The IRS, and the question of whether it singled out conservative groups applying for nonprofit status, has been a subject of investigation by the committee for months.

Cummings, who has made no secret of his distaste for True the Vote, denied that his staff made any inappropriate requests. The emails show aides had requested "publicly available information." Other documents have revealed the agency was already looking into True the Vote months before Democratic staff made the request.

"We wanted to find … only public records — that's the only thing we've asked for, we would not have wanted anything else and that's the only thing we got — to see exactly what the deal was with … True the Vote," Cummings said.

True the Vote founder Catherine Engelbrecht told Fox News in February that she filed a complaint with the Office of Congressional Ethics against Cummings for trying to intimidate her. A spokeswoman for the ethics office declined to comment.

"There's no jurisdiction of this committee into a private citizens group — it's just a complete abuse of power," said Cleta Mitchell, a lawyer who represents the group. "It's just bullying."

Cummings, who was first elected to Congress in 1996, represents parts of Baltimore City and Baltimore and Howard counties. Before his time in Washington, he spent 13 years in the Maryland General Assembly.

David Leviss, who served as a staff member to the oversight committee under Democrats, said relations between its chairmen and its ranking members have always been contentious.

But in the past, he said, the majority and the minority were able to work together on more issues — such as contracting fraud — which gave lawmakers and staff a chance to build trust. Much of that bipartisan spirit seems to have waned, Leviss said.

"I don't think the friction between Issa and Cummings is unique," said Leviss, who is now an attorney at O'Melveny & Myers. "What's different now is that there doesn't seem to be very much common ground."

Issa faces a term limit as committee chairman at the end of this year and several Republicans are jockeying for his seat. It's not yet clear whether Cummings will stay on as the top Democrat after this year's midterm elections.

But despite the bruising fights, Cummings says he'd like to.

"If they ask me to," he said, "I'll stay."