The House speaker, Nancy Pelosi, is readying the committee that will on Tuesday begin its investigation into the attack on the Capitol to press ahead with an aggressive inquiry into Donald Trump, as she seeks to exploit Republicans’ refusal to participate that could leave the former US president unguarded.

The move by the top Democrat in the House last week to block Jim Banks and Jim Jordan – vociferous allies of the former president – from serving on the House select committee, prompted the House minority leader, Kevin McCarthy, to boycott the inquiry, pulling his three other Republican picks from the panel.

But Pelosi won strong support from Democrats for her vetoes and told her lieutenants that she may have emerged with the upper hand ahead of the select committee’s first hearing.

“We have the duty, to the constitution and the country, to find the truth of the 6 January insurrection and to ensure that such an assault on our Democracy cannot again happen,” Pelosi said of the investigation in a letter to Democrats.

The speaker has suggested to top Democrats in recent days that McCarthy’s reflexive move to boycott the panel leaves Trump without any defenders in the high-profile investigation into the 6 January insurrection, according to a source familiar with the matter.

Pelosi chose some of the former president’s most critical opponents when she made her appointments to the select committee, installing both lead House impeachment managers from Trump’s two impeachments as well as the Republican dissident Liz Cheney, who was ousted from party leadership in May for repudiating Trump.

But the absence of any Republican picks on the select committee means that when the investigation pivots from examining security failures to the role Trump played on 6 January, the inquiry will be conducted solely by his political foes, emboldening Pelosi to seek an aggressive inquiry, the source said.

It was not immediately clear how Pelosi might proceed with the select committee. But Democrats have agitated for weeks for her to take a hard line against Republicans after they doomed a 9/11-style bipartisan commission into the Capitol attack.

Democrats close to Pelosi say she remains furious at how Republicans have sought to downplay the brutal violence of the insurrection, which informed her decision to not give Banks and Jordan a platform from which to twist or minimize the select committee’s findings.

But the speaker’s relationship with Republicans deteriorated to a new low, after McCarthy shouted down the phone at her when she informed him of her decision to veto Banks and Jordan, the source said.

Republicans have seized on Pelosi’s intervention against Banks and Jordan, as well as her close involvement with the panel, to portray the inquiry as a partisan exercise to gain political advantage ahead of the 2022 midterm elections.

McCarthy lashed out anew a day after he was denied his two top picks for the select committee, and pledged to carry out a Republican-only investigation that would focus on how Pelosi should have done more to protect the Capitol.

But Democrats said Pelosi was more than justified in upending congressional norms in refusing to appoint Banks and Jordan, both of whom amplified Trump’s lies about a stolen election and objected to certifying Joe Biden’s election win.

“We want people who are going to have allegiance to the oath of office that they took, not an allegiance to one person. And they’ve clearly pledged their allegiance to the former president,” said Democrat Pete Aguilar, a member of the select committee.

Several Democrats said they were particularly disturbed by a CNN report that an alleged Capitol rioter, Anthony Aguero, accompanied Banks on a trip sponsored by the Republican Study Committee to the southern border and, at times, served as an interpreter.

They also said that Pelosi came to the conclusion that Banks could not be trusted to serve as the top Republican appointee on the panel after he issued a statement that he wanted to investigate the role of the Biden administration in the insurrection.

Democrats expressed deep reservations as well about Jordan, the top Republican on the House judiciary committee, after he disparaged the select committee and accused Pelosi of being responsible for a diminished security presence at the Capitol.

The speaker does not herself oversee security at the Capitol, which is the responsibility of the US Capitol police board and the House and Senate sergeants-at-arms. Both sergeants-at-arms at the time of the attack were hired by Republican congressional leaders.

A bipartisan Senate report released last month detailed multiple security failings on the parts of the US Capitol police and the sergeants-at-arms. It did not blame Pelosi or her then opposite number in the Senate, Mitch McConnell.