Over half of House Democrats now support opening an impeachment inquiry into President Donald Trump, marking the first time a majority of the caucus is on record in favor of such an action and increasing the pressure on Speaker Nancy Pelosi to back the move.

"That's momentum," tweeted activist group Stand Up America. "We must keep it up."

Rep. Ted Deutch (D-Fla.) proved the tipping point, announcing his support Thursday morning in an opinion piece for Florida's Sun Sentinel newspaper. The representative cited the Judiciary Committee's investigations into the president as evidence that the subpoena powers of Congress mean an inquiry has already begun.

"The stated purpose was to consider all constitutional remedies for presidential misconduct, including impeachment," wrote Deutch. "In every meaningful way, our investigation is an impeachment inquiry."

As Politico reported Thursday, Deutch puts the number of Democrats in favor of impeachment at 118 and indicates that the appeal of impeachment is going beyond the party's activist and left-leaning wings to moderate members in swing districts.

Perhaps more significant than the raw number of Democrats backing an inquiry is the identities of the members themselves. The latest additions include Reps. Mike Levin (D-Calif.), Chris Pappas (D-N.H.), Jennifer Wexton (D-Va.) and Jason Crow (D-Colo.), four freshmen who flipped Republican-held districts in November. Rep. Katherine Clark (D-Mass.), a member of Pelosi's leadership team, added her name to the list on Friday.

"It's a significant milestone that backers hope will change the politics inside the Democratic caucus even as Pelosi continues to resist," said reporter Kyle Cheney.

With an ideologically diverse majority of the caucus on board, it may prove difficult for Pelosi, a California Democrat, to stop the party from going after the president. Pelosi has tried to tamp down her party's calls for impeachment, citing a desire to have the strongest possible case before bringing the articles to a trial in the Senate, where the president is almost certain to be acquitted.

In a New York Times analysis, reporter Nicholas Fandos noted that the Speaker and House leadership want to at least slow the process down:

Ms. Pelosi and her top lieutenants remain skeptical of advancing a full-bore impeachment without broader public support and are steering the caucus forward with one foot tapping the brakes. They want to see if the House can use the courts to free up information and witnesses related to Robert S. Mueller III's investigation that are being blocked by the White House before reaching conclusions—a process that could take months, at best.

"With more than half of House Democrats now publicly backing impeachment proceedings, Pelosi's efforts to stem the tide of a full-fledged revolt against her 'wait and see' approach look more imperiled than ever," wrote Splinter's Rafi Schwartz.

Slowing or stopping the march toward impeachment may prove difficult in the August recess. Progressive groups plan to harry Democrats in their home districts and demand the representatives take action against the president. The new activist-led campaign, BuzzFeed News reported Thursday, is intended to force lawmakers on the fence to reconsider their position.

The groups—Indivisible, MoveOn, Need to Impeach, and Stand Up America—told BuzzFeed News exclusively that the joint campaign will focus on members of House leadership and Judiciary Committee members who don't yet support an inquiry—including committee chair Jerry Nadler—though they hope to connect volunteers with every member of Congress, including Republicans, to talk about the issue.

"People understand that no one should be above the law in this country," Stand Up America founder Sean Eldridge told BuzzFeed. "If any other American obstructed justice as Donald Trump did, they would be in jail."