Can impeachment managers be changed once the Senate trial starts?

Question

So Pelosi has picked the "impeachment managers" for the House.

Are there rules that these managers can't be changed once the trial in the Senate starts? If so, what is the rationale for preventing a change?

Answer 1

It's important to remember that each chamber sets its own rules for impeachment. As such, the answer is entirely in how each chamber votes

1. There's no Constitutional mandate for "impeachment managers". That is a convention of the House. In theory, the House may change its selected managers any time it sees fit by holding a vote to change them.
2. The Senate does not need House managers to carry out the trial. It's entirely possible that an attempt to replace managers by the House would be met with the Senate rejecting them entirely and proceeding without them.

Answer 2

Are there rules that these managers can't be changed once the trial in the Senate starts? If so, what is the rationale for preventing a change?

There are no specific rules, nor is there any precedent, for changing managers during an impeachment trial.

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The House managers have been appointed in different ways.

Selection of by House:, [p. 59]

In the trial of Charles Swayne, the Speaker of the House was authorized to appoint seven managers, four of whom belonged to the majority party, and three to the minority. Five of seven were members of the Judiciary Committee.

In other cases, the managers have been chosen by ballot. This was done in the Belknap case, the Blount case, the Pickering case, the Chase case, the Peck case, and the Johnson case. The most recent practice has been to adopt a resolution in the House of Representatives naming the managers on the part of the House.

In the case of a resolution, adding or changing the managers during the trial would possibly, but not necessarily, require a amended resolution by the House. At the very least, it would require a message from the House notifying the Senate of the change.

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The Senate rules require recognition of the appointed managers at the start of the trial proceedings. To change the House managers at a later time may require a vote in the Senate to accept or reject the change; because the change is not addressed by rule or precedent.

II. RULES OF PROCEDURE AND PRACTICE IN THE SENATE WHEN SITTING ON IMPEACHMENT TRIALS, [p. 2]

I. Whensoever the Senate shall receive notice from the House of Representatives that managers are appointed on their part to conduct an impeachment against any person and are directed to carry articles of impeachment to the Senate, the Secretary of the Senate shall immediately inform the House of Representatives that the Senate is ready to receive the managers for the purpose of exhibiting such articles of impeachment, agreeably to such notice.

II. When the managers of an impeachment shall be introduced at the bar of the Senate and shall signify that they are ready to exhibit articles of impeachment against any person, the Presiding Officer of the Senate shall direct the Sergeant at Arms to make proclamation, [...]; after which the articles shall be exhibited, and then the Presiding Officer of the Senate shall inform the managers that the Senate will take proper order on the subject of the impeachment, of which due notice shall be given to the House of Representatives.

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However, articles of impeachment have been amended during the impeachment trial. Thus, there appears to be no reason why the House managers could not be changed during the trial.

Amendments to: [pp. 37-38]

In the trial of Halsted L. Ritter, the House of Representatives amended their original articles of impeachment. On March 30, 1936, they sent the following message to the Senate:

Resolved, That a message be sent to the Senate by the Clerk of the House informing the Senate that the House of Representatives has adopted an amendment to the articles of impeachment heretofore exhibited against Halsted L. Ritter, United States district judge for the southern district of Florida, and that the same will be presented to the Senate by the managers on the part of the House.

And also that the managers have authority to file with the Secretary of the Senate, on the part of the House, any subsequent pleadings they shall deem necessary.

The following day, March 31, the amendments to the articles were presented, by the managers on the part of the House, and the counsel for the respondent asked for 48 hours to file his response to the new articles.

In the case of Judge Harold Louderback in 1933, article V of the article of impeachment was amended by the House of Representatives. The following proceedings occurred:

Resolved, That a message be sent to the Senate by the Clerk of the House informing the Senate that the House of Representatives has adopted an amendment to article V of the articles of impeachment heretofore exhibited against Harold Louderback, United States district judge for the northern district of California, and that the same will be presented to the Senate by the managers on the part of the House,

And, also that the managers have authority of file with the Secretary of the Senate, on the part of the House, any subsequent pleadings they shall deem necessary.

Mr. Sumners, on behalf of the managers on the part of the House, presented article V of the articles of impeachment, as amended, and proceeded to read the same; when,

On motion by Mr. Ashurst, and by unanimous consent, The reading of the said article, as amended, was dispensed with, and it was ordered to be printed for the use of the Senate.