In the Thomases, the committee is up against a couple that has deep networks of support across the conservative movement and Washington, including inside the committee. The panel’s Republican vice chairwoman, Representative Liz Cheney of Wyoming, has led the charge in holding Mr. Trump to account for his efforts to overturn the election, but has wanted to avoid any aggressive effort that, in her view, could unfairly target Justice Thomas, the senior member of the Supreme Court.

So although a debate has broken out inside the committee about summoning Ms. Thomas to testify, the panel at this point has no plans to do so, leaving some Democrats frustrated. That could change, however: On Friday, despite the potential for political backlash, Ms. Cheney indicated she has no objection to the panel asking Ms. Thomas for a voluntary interview.