House Speaker Nancy Pelosi sharpened the focus of Democrats’ impeachment case against President Donald Trump on Thursday, accusing the president of committing bribery when he withheld vital military assistance from Ukraine at the same time he was seeking its commitment to publicly investigate his political rivals.

The speaker’s explicit allegation of bribery, a misdeed identified in the Constitution as an impeachable offense, was significant. Even as Pelosi said that no final decision had been made on whether to impeach Trump, it suggested that Democrats are increasingly working to put a name to the president’s alleged wrongdoing and moving toward a more specific set of charges that could be codified in articles of impeachment in the coming weeks.

Democrats have begun using the term “bribery” more freely in recent days to describe what a string of diplomats and career Trump administration officials have said was a highly unusual and inappropriate effort by Trump and a small group around him to extract a public promise from Ukraine to investigate former Vice President Joe Biden and a discredited theory about Democrats conspiring with Ukraine to interfere in the 2016 election.

Over lunch at the White House, Trump, working to defend himself and shore up his support among Republicans in the face of an existential threat to his presidency, showed a group of senators a reconstructed transcript of a congratulatory phone conversation he had in April with President Volodymyr Zelenskiy of Ukraine. He has promised repeatedly to release the document publicly, in part to counter the notion that he ever pressured Zelenskiy.