The drumbeat for impeachment is growing. Can Nancy Pelosi stop it?

From the start, Speaker of the House Nancy Pelosi has resisted calls to impeach President Donald Trump.

She’s called the move divisive. She refused to budge even as rumblings grew among House Democrats that special counsel Robert Mueller’s report had enough to start impeachment proceedings against Mr. Trump on obstruction of justice grounds. On Monday night, Ms. Pelosi told Democratic leaders that impeachment would take away from the party’s legislative agenda and undercut ongoing investigations into the president by other congressional committees.

But the pressure is mounting, as the Trump administration continues blocking Democrats’ every effort at investigation. It’s unclear how long can Ms. Pelosi can hold the line.

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“The more they stonewall, the more powerless people feel, and the more they want to take this immediate action,” says Krystal Ball, a political commentator and host on The Hill’s digital news channel. “The pressure comes from that sense of powerlessness and disgust.”

An April CNN poll found that 69% of Democratic voters want Congress to move forward with impeachment. Billionaire Tom Steyer this month launched a $1 million ad campaign accusing Democrats of doing nothing while the president got away with obstruction and corruption. Even Rep. Justin Amash of Michigan, a Republican and member of the conservative Freedom Caucus, said over the weekend that the president should be impeached for obstruction of justice.

All the while, Mr. Trump has stayed on war footing with congressional Democrats, almost daring them to go ahead. “If it’s an impeachment proceeding, then somebody should call it that,” Rudy Giuliani, one of the president’s personal lawyers, told The New York Times this month. “If you don’t call their bluff now, they’ll just keep slithering around for four, five, six months.”

At the Monday meetings, some Democratic leaders pressed Ms. Pelosi on the issue, arguing that impeachment proceedings would give them access to the documents that the administration has refused to provide. The situation intensified Tuesday after former White House counsel Don McGahn failed to obey a Judiciary Committee summons. Even members less explicitly in favor of taking that step say the White House is pushing them toward doing so.

“The initial aim was to investigate and then see what we have. The problem is we can’t get any information,” Maryland Rep. Elijah Cummings, chair of the House Oversight Committee, tells reporters. “The caucus is slowly moving toward saying, ‘Well, what do we do?’ Because he’s not leaving us with any choices.”

This group of lawmakers – reluctant to move toward impeachment but also increasingly willing to take that step – could be the tipping point for Ms. Pelosi, says Kris Miller, who teaches American government and legislative politics at the University of Maryland, College Park. Before this week, the loudest calls for impeaching the president had come mostly from progressive members in solidly blue districts, she says. Now, some combination of institutional duty and being boxed in by the administration’s stonewalling has caused a rise in this third camp.

“Their numbers are not huge, but they’re growing quickly,” Professor Miller says. “I think that changes a lot for the caucus overall.”

Still, Ms. Pelosi isn’t entirely out of options – yet. Democrats have managed to put the Trump administration on the defensive in one court case: On Monday, a federal district court judge ruled that the president’s accounting firm must turn over his financial records to the House Oversight Committee. Judge Amit Mehta rejected the Trump legal team’s argument that the committee had no authority to investigate the president.

There’s also a view that if Ms. Pelosi could hold her caucus together until a little closer to the 2020 primaries – if the burst of voices calling for impeachment dies down in the next day or two – she might be able to more successfully make the case that the best way to defeat Mr. Trump is through the ballot box.

And while some Democratic presidential candidates have voiced support for impeachment, “It’s not absolutely in their interest to have all this going on while they’re trying to introduce themselves to the public,” says John Fortier, of the Bipartisan Policy Center. “The argument that impeachment may not be good for [the Democratic Party] politically is a strong one.”