“At the very same time a global crisis was unfolding in real time, she published another ‘dear colleague’ letter saying she intends to keep our commander in chief in this limbo indefinitely,” Mr. McConnell said.

The pointed remarks were the latest in an escalating confrontation between two of the most powerful leaders Congress has seen in a generation.

Mr. McConnell, 78, and Ms. Pelosi, 79, each of whom has served for more than three decades, are considered master tacticians in their parties, and neither is known for backing down from a contentious fight. Both have a keen understanding of the political dynamics in their rank and file, and excel at consolidating their colleagues around a single position and refusing to budge.

Mr. McConnell’s objective in the current battle is to bring about a speedy acquittal of the president, belittling the House’s case in the process. He went to the White House Wednesday afternoon on other business, but spoke with the president about the impending trial, according to a person with knowledge of the meeting. Democrats have already accused the majority leader of too closely coordinating with the defense.

Ms. Pelosi, having carefully orchestrated the impeachment vote in the House, does not want to allow the Senate to quickly bury the matter without delving into additional witness accounts or documents. And if she cannot force Mr. McConnell to agree to those terms, she is determined at least to convince the public that the Senate trial is illegitimate — and, by extension, that Mr. Trump’s acquittal was rigged from the start.

The two leaders, whose relationship has grown chillier in recent years, have not spoken directly about impeachment stalemate, instead carrying out their terse exchanges in speeches and through the news media.

The latest comments from Mr. McConnell came in response to a letter from Ms. Pelosi to House colleagues late Tuesday in which she accused him of putting his loyalty to the president above the Constitution. In recent weeks, she has also referred to Mr. McConnell as “rogue” and complicit in a cover-up of what she described as the president’s misbehavior.