Now the two giants of Capitol Hill have each other in their sights, as Democrats and Republicans feud over the terms of a Senate trial of President Donald Trump.
The speaker's refusal to immediately transmit the articles of impeachment passed by the House to the Senate has triggered a rare direct power game with the Senate majority leader, pitting two of American history's strongest congressional figures against each other in a battle not likely to cool off over a winter recess on Capitol Hill.
"We remain at an impasse on these logistics," McConnell said on the Senate floor after a meeting with Senate Minority Leader Chuck Schumer and moments before informally closing out the chamber for the next two weeks.
Earlier Thursday, McConnell had jabbed at Pelosi, suggesting that her decision to hold on to the articles, apparently designed to offer leverage to Schumer in bargaining over the shape of a trial, showed weakness.
"Speaker Pelosi suggested that House Democrats may be too afraid, too afraid to even transmit their shoddy work product to the Senate," the Kentucky Republican said, with a politically shrewd but highly selective interpretation of the Democratic position.
The response from the California Democrat, who was raised in a politically oriented Baltimore family, was scathing and apparently premeditated.
"I heard some of what Mitch McConnell said today, and it reminded me that our founders ... when they wrote the Constitution, they suspected that there could be a rogue president," Pelosi said.
She added, "I don't think they suspected that we could have a rogue president and rogue leader in the Senate at the same time."
The clash came as talks between McConnell and Schumer, a New York Democrat, reached an stalemate over the shape of the trial, leaving the chaotic impeachment battle in suspended animation between the two chambers as lawmakers emptied out of the Capitol for a two-week holiday break.
Masters of Washington politics
There's an argument that Pelosi and McConnell, ideological opposites but masters of their respective political lairs, are the most consequential political leaders in a period that encompassed the presidencies of George W. Bush, Barack Obama and Trump.
Pelosi, the highest-ranking woman in the history of the US government, won one speakership in 2006 and later used it to pass Obamacare, the most sweeping social legislation in decades. She paid with her majority but won her gavel back in another midterm election triumph helped by an unpopular Republican commander in chief and, reluctantly, impeached a president.
McConnell has been a rock of stability in the wild Trump era, navigating the squalls of a riotous, undisciplined President who contrasts sharply with his languid manipulation of the levers of power. He has methodically built the most conservative cadre of federal judges in history. His earlier ruthless power move in refusing to confirm Obama's Supreme Court pick Merrick Garland outraged his foes. But it turned out to be one of the most significant Washington gambits in decades, as political tectonics realigned and McConnell enshrined a robust conservative majority on the nation's top bench.
Pelosi knows a bit about facing down presidents too. She bluntly told Trump not to show up for his State of the Union address earlier this year during a government shutdown. Even before she reclaimed the speaker's chair, she rebuked Trump on television in the Oval Office, reflecting the way she has become one of the few Washington players to regularly outmaneuver the President.
Pelosi and McConnell go head to head
Both Pelosi and McConnell have an almost supernatural understanding of political forces at work on their caucuses. Both consider their moves carefully but can be outspoken and make news. They rarely make mistakes. Both revere the chambers they lead. And recently, each has begun firing more scathing remarks across Statutory Hall.
In August, frustrated at McConnell's decision to block election interference legislation and over the House bills that die on his desk, the speaker borrowed a Russia-themed insult that had previously offended the majority when used by pundits.
"Moscow Mitch says that he is the 'Grim Reaper,' " Pelosi said at the time. "Imagine describing yourself as the 'Grim Reaper' -- that he's going to bury all this legislation."
McConnell, whose "Reaper" nickname references in part the House-passed legislation he's allowed to pile up at his door over the year, has also seemed to damn Pelosi with faint praise and to use her as a useful foil -- on immigration, for instance -- as he seeks to show he is completely in line with Trump and the GOP base on key issues.
"The new speaker of the House, the very distinguished congresswoman from San Francisco, has decided that opposing President Trump comes before the security of our borders," McConnell said in a Senate floor speech on January 14.
The tussle over the trial
After the House on Wednesday made Trump only the third impeached president, Pelosi said she was holding out to ensure that the GOP holds a fair Senate trial early next year.
She eased back from a demand for witnesses on Thursday, but nevertheless the House left town without handing over the articles, meaning the question will linger over the holiday.
Pelosi may not have much leverage. McConnell, like the speaker herself, is not one to climb down. And the idea that the Senate could allow itself to be forced into any step by what many of its incumbents view as decidedly the junior chamber seems unlikely.
Pelosi's move may marginally improve Schumer's negotiating position on the arrangements for the trial. But she may be playing a mind game with another of her adversaries -- the President himself.
She now keeps Trump, who is keen for a swift trial and acquittal that he can brand as absolution, fretting for the entirety of his seasonal break in Florida.
The President already seems agitated.
According to a top Senate ally, Lindsey Graham of South Carolina, the commander in chief asked Thursday: "What are they doing?"
Graham said he replied: " 'Mr. President, I don't know.' "
That night, the President tweeted: "So after the Democrats gave me no Due Process in the House, no lawyers, no witnesses, no nothing, they now want to tell the Senate how to run their trial. Actually, they have zero proof of anything, they will never even show up. They want out. I want an immediate trial!"
Danny Weiss, former chief of staff for Pelosi, said delaying sending the articles of impeachment gave the speaker more time to consider who she wanted to name as the impeachment managers who will present the House case in the Senate trial.
And a pause allows warnings by McConnell and Graham that they will not serve as impartial jurors to create a political backlash, he said.
"By delaying, she is giving a little more time for the public and others to examine what that process should look like," Weiss told CNN's Brooke Baldwin.
But just as she did during the impeachment investigation itself, Pelosi must be conscious of the impact of prolonging the process on vulnerable House Democrats who face tough reelection races in districts where Trump won big in 2016.
One of those lawmakers, Rep. Elissa Slotkin of Michigan, told CNN's Manu Raju on Thursday that it "seems reasonable" to ask for a fair process in the Senate but that it "can't drag on forever."
The House broke up for the holidays after a tumultuous year without voting on naming impeachment managers on Thursday evening. That means the issue of the stalled articles will hang over the holidays for sure.