Democratic Speaker Nancy Pelosi decided to block two Republican House members from the house select committee formed to investigate on January 6th. The Republican House Leader Kevin McCarthy has characterized her actions as "unprecedented" and an "egregious abuse of power":
Speaker Nancy Pelosi has taken the unprecedented step of denying the minority party’s picks for the Select Committee on January 6. This represents an egregious abuse of power and will irreparably damage this institution. Denying the voices of members who have served in the military and law enforcement, as well as leaders of standing committees have made it undeniable that this panel has lost all legitimacy and credibility and shows the Speaker is more interested in playing politics than seeking the truth.
Source: Republican leader McCarthy's Statement about Pelosi’s Abuse of Power on January 6th Select Committee dated July 21, 2021
Is there, in fact, any historical precedent for Pelosi's action?
Some answers and comments have pointed out Nancy Pelosi's justification of her action, saying: “The unprecedented nature of January 6th  demands this unprecedented decision.”
I take the point of this. However, from a historical perspective, her justification seems factually incorrect. Other comments suggested other cases of violence at the Capitol. But I think the following is most relevant counterexample:
On June 21, 1783, eighty soldiers from Lexington, MA were unpaid and weary; they marched on the Congress sitting in Philadelphia. The angry mob was joined 300 others from the Philadelphia barracks. Together, they physically threatened and verbally abused the members and caused them to flee the city. See Congress Flees to Princeton 1783. See also J. Fiske, The Critical Period of American History, 1783–1789 112–113 (1888); W. Tindall, The Origin and Government of the District of Columbia 31–36 (1903).
1783 was probably the most historically significant example of threats of violence against congress precisely because it occurred before the modern Constitution was adopted. Unlike 1812, it was an armed American militia who sought to protest their treatment by the government. Unlike 2021, the incident was resolved without the use of deadly force.
The government eventually paid the militias, and U.S. Constitution, Article I, Section 8, Clause 17 was adopted after the incident to grant the government the authority to create the District of Columbia (now called Washington DC); that was the clearest impact of the incident having been investigated. It is also worth noting how the incident was resolved:
Mr. Reed moved that the General should endeavor to withdraw the troops by assuring them of the disposition of Congress to do them justice. … In the meantime, the soldiers remained in their position, without offering any violence, individuals only occasionally uttering offensive words and wantonly pointed their Muskets to the Windows of the Hall of Congress. No danger from premeditated violence was apprehended, but it was observed that spirituous drink from the tippling houses adjoining began to be liberally served out to the Soldiers, & might lead to hasty excesses.
Source: the Journal of the Third Continental Congress
Thus the founding fathers resolved it without all the deaths and partisan fingerpointing seen today.