WASHINGTON — The House committee investigating the Jan. 6 attack on the Capitol subpoenaed two of Donald J. Trump’s campaign aides and Republican Party officials from battleground states on Tuesday as it dug deeper into a plan to use false slates of electors to help the former president stay in office after he lost the 2020 election.

The use of bogus slates of electors was one of the more audacious gambits used by allies of Mr. Trump to try to keep the presidency in his hands, and the committee’s members and investigators have made it increasingly clear in recent days that they believe the effort — along with proposals to seize voting machines — was a major threat to democracy.

Among those subpoenaed on Tuesday were Michael A. Roman and Gary Michael Brown, who served as the director and deputy director of Election Day operations for Mr. Trump’s campaign. The panel also summoned Douglas V. Mastriano, a Pennsylvania state senator; Laura Cox, the former chairwoman of Michigan’s Republican Party; Mark W. Finchem, an Arizona state legislator; and Kelli Ward, the chairwoman of Arizona’s Republican Party.

In letters accompanying the subpoenas, the committee said it had obtained communications that showed Mr. Roman’s and Mr. Brown’s “involvement in a coordinated strategy to contact Republican members of state legislatures in certain states that former President Trump had lost and urge them to ‘reclaim’ their authority by sending an alternate slate of electors that would support former President Trump.”

“It appears that you helped direct the Trump campaign staffers participating in this effort,” Representative Bennie Thompson, Democrat of Mississippi and the chairman of the committee, wrote to Mr. Roman.

The committee said that Mr. Finchem, who was on the Capitol grounds on Jan. 6, was in communication with leaders from the “Stop the Steal” movement regarding a rally at the Capitol, and that Mr. Finchem said he was in Washington to “deliver an evidence book and letter to Vice President Pence showing key evidence of fraud in the Arizona presidential election, and asking him to consider postponing the award of electors.”

In its letter to Ms. Cox, the panel said it had evidence that she witnessed Rudolph W. Giuliani, Mr. Trump’s personal lawyer, pressure state lawmakers to disregard the election results in favor of Joseph R. Biden Jr. in Michigan and say that certifying the election results would be a “criminal act.”

After the November election was over, Ms. Ward sent a message to an Arizona elections official warning to “stop the counting,” according to the committee. She also “apparently spoke with former President Trump and members of his staff about election certification issues in Arizona” and “posted a video advancing unsubstantiated theories of election interference by Dominion Voting Systems along with a link to a donation page to benefit the Arizona Republican Party,” the committee said.

Ms. Ward also claimed to be an “alternate” elector for Mr. Trump, even though Mr. Biden won Arizona.

The committee said Mr. Mastriano spoke directly “with former President Trump about your postelection activities.” Mr. Mastriano, a former Army officer, was also on the Capitol grounds on Jan. 6, though he later explained in a statement that “he followed the directions of the Capitol Police and respected all police lines” that day.

The subpoenas instruct the witnesses to produce documents and sit for depositions in March.

“The select committee is seeking information about efforts to send false slates of electors to Washington and change the outcome of the 2020 election,” Mr. Thompson said, adding, “The select committee has heard from more than 550 witnesses, and we expect these six individuals to cooperate as well as we work to tell the American people the full story about the violence of Jan. 6 and its causes.”

The scheme to employ the so-called alternate electors was one of Mr. Trump’s most expansive efforts to overturn the election, beginning even before some states had finished counting ballots and culminating in the pressure placed on Mr. Pence to throw out legitimate votes for Mr. Biden when he presided over the joint congressional session to finalize the election outcome.

At various times, the gambit involved lawyers, state lawmakers and top White House aides.

The New York Times reported this month on legal memos that show some of the earliest known origins of what became the rationale for the use of alternate electors.

The memos — from a lawyer named Kenneth Chesebro to James R. Troupis, a lawyer for the Trump campaign in Wisconsin — show how, just over two weeks after Election Day, Mr. Trump’s campaign was seeking to buy itself more time to undo the results. At the heart of the strategy was the idea that their real deadline was not Dec. 14, when official electors would be chosen to reflect the outcome in each state, but Jan. 6, when Congress would meet to certify the results.

The two memos were used by Mr. Trump’s top lawyer, Mr. Giuliani, and others like John Eastman as they developed a strategy intended to exploit ambiguities in the Electoral Count Act, according to a person familiar with the matter.

The subpoenas are the latest step the committee has taken to investigate the plans to use electors who falsely attested Mr. Trump had won their states.

Last month, the committee issued 14 subpoenas to people who claimed to be electors for Mr. Trump in states that were actually won by Mr. Biden. Those subpoenas targeted individuals who met and submitted pro-Trump Electoral College certificates in seven states Mr. Biden won: Arizona, Georgia, Michigan, New Mexico, Nevada, Pennsylvania and Wisconsin.

Those who signed on to the fake slates of electors were mostly state-level officials in the Republican Party, G.O.P. political candidates or party activists involved with Mr. Trump’s re-election campaign.

Those subpoenas came after the Justice Department said it was investigating the fake electors.

Ultimately, Mr. Pence rejected plans to throw out the legitimate electoral votes in favor of those false slates for Mr. Trump.

Marc Short, Mr. Pence’s chief of staff, recently testified privately before the committee. In a television interview this month, he said Mr. Pence was aware of the so-called alternate electors, but did not believe it was appropriate to accept slates not chosen by the voters.

“There were discussions about alternative slates in certain letters that we received,” Mr. Short told NBC. “But, you know, when we had a conversation with the parliamentarian, she made it clear that, candidly, every year they receive notes from random Americans saying, ‘Here’s my slate of electors.’ Unless they’re certified by the state, they’re candidly meaningless.”

Alan Feuer contributed reporting.