When Democrats declared in 2019 that they were moving ahead with an impeachment inquiry into President Donald J. Trump without a House vote, Representative Kevin McCarthy, then the minority leader, railed against the move.

“Speaker Pelosi can’t decide on impeachment unilaterally. It requires a full vote of the House of Representatives,” he wrote on Twitter.

The California Republican, now speaker himself, reiterated that point of view just days ago, telling the conservative news outlet Breitbart that if Republicans were to move forward with an impeachment inquiry against President Biden, it would be by a vote — “not through a declaration by one person.”

But this week, Mr. McCarthy made just such a declaration, directing three committees to begin an impeachment inquiry without approval by the full House.

Mr. McCarthy’s decision reflected the substantial political pressure he is facing from right-wing members of Congress who have demanded that he move forward quickly and aggressively with an impeachment inquiry and deep spending cuts or risk a government shutdown, and who have threatened to depose him if he fails to do their bidding.

It also reflected his awareness that he currently lacks the votes among his fellow Republicans to approve an impeachment inquiry, because more centrist members of the party have expressed discomfort about moving forward in the absence of solid evidence against Mr. Biden.

As a justification, the speaker has pointed to the actions of Ms. Pelosi — who announced a formal impeachment inquiry against Mr. Trump in 2019 five weeks before the House voted to authorize one — as setting a precedent that he is following.

At the Capitol on Wednesday, he said he wasn’t bothered by contradicting his previous stance. “Nancy Pelosi changed the rules and the precedent,” he said.

And he declined to answer questions about what had changed since 11 days ago, when he told Breitbart News: “If we move forward with an impeachment inquiry, it would occur through a vote on the floor of the people’s House and not through a declaration by one person.”

There are no binding rules about what the House must do to begin an impeachment inquiry, and history offers no clear guide. The House Judiciary Committee began an impeachment investigation into President Richard M. Nixon in October 1973, but did not vote to ratify a formal inquiry until February 1974. In 1998, the House voted to open an impeachment inquiry into President Bill Clinton. In 2021, the House impeached Mr. Trump a second time with no inquiry at all.

But pursuing an impeachment inquiry without a vote of the full House creates a scenario in which some targets of subpoenas could put forth legal arguments to resist complying. A 2020 Justice Department memo written during the Trump administration argued that an impeachment inquiry was invalid without a vote of the House. It is not yet clear whether the Biden White House will make a similar argument.

“Speaker McCarthy’s announcement of an impeachment inquiry by relevant committees raises interesting questions about authority to issue and enforce subpoenas in pursuance of the inquiry,” said Stanley Brand, the former House general counsel. “It is clear that authority to issue subpoenas comes from the full House, and with respect to impeachment, that requires authority from the House.”

Regardless of the legal issues, Democrats were quick to condemn the about-face by Mr. McCarthy.

“The hypocrisy almost takes your breath away. Kevin McCarthy has repeatedly said you cannot start an impeachment inquiry without a full vote of the House,” said Representative Ilhan Omar, Democrat of Minnesota, noting that Mr. McCarthy had gone so far as to introduce a resolution in 2019 that condemned an impeachment inquiry without a House vote as an “abuse of power” that “brings discredit to the House.”

“Yet that is exactly what Kevin McCarthy is doing today,” she added. “Why? Because he knows impeachment is too extreme to pass the House — even with the Republican majority.”

In 2019, Mr. McCarthy’s resolution railed against what he called Ms. Pelosi’s “extraordinary decision to move forward with an impeachment inquiry without any debate or vote,” and argued that such an action “undermines the voting privileges afforded to each member and the constituents they represent.”

All that appeared to have changed by Tuesday.

In many regards, the announcement of an impeachment inquiry has less to do with the investigatory powers of Congress and more to do with the politics that take place under the Capitol Dome. Three House committees are already investigating the Biden family and the president’s administration, and the impeachment inquiry is mostly a rebranding exercise and an extension of those existing efforts.

The committees have been successful in obtaining a wide array of documents and hauling in dozens of witnesses without the need to fight protracted litigation. The Oversight Committee, led by Representative James R. Comer, Republican of Kentucky, has already received more than 12,000 pages of subpoenaed bank records, reviewed more than 2,000 pages of suspicious activity reports and spent hours interviewing witnesses, including two of Hunter Biden’s former business associates.

But Mr. McCarthy argued he needed a greater justification to issue subpoenas for the bank records of a president and his family.

“An impeachment inquiry is simply empowering the House to a greater level to get the documents,” he told reporters, adding: “We don’t have the president’s bank statements. We don’t have Hunter Biden’s bank statements.”