I’m curious what you imagine the objection to be.
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As already explained, the decision to testify or not is exclusively that of the Defendant, not the attorney (this is an exception from the usual rule that the lawyer makes the tactical decisions, such as who to call as a witness and what questions to ask). This is, in fact, emphasized at trial - usually, at some break in the case, the court will advise the Defendant of this right, but not actually require the Defendant to make the decision until right before it would be relevant (e.g. after the prosecutor has rested the state’s case, and it is now up to the defense to decide if they will call witnesses).
Usually, a defense attorney will counsel against having the witness testify. Why? Lots of reasons:
For one, most people accused of crimes are, in fact, guilty. Why put a person on the stand who clearly did the deed, so that the prosecutors can pepper them with questions? There’s no benefit.
Besides, testifying under oath can be very stressful. Many defendants are not well educated. They may be excitable, or have a temper. These things are not necessarily relevant to the determination of guilt, but they will definitely color the jury’s feelings about the case. (As my boss once said about a celebrity client, better to have him sit at the table and let the jury think all sorts of things about him then have him testify and ‘ruin the illusion’ of his public persona).
Also, once the defendant testifies, he’s opened himself up to the prosecutor calling rebuttal witnesses to rebut whatever story he’s told.
And, if he has prior felonies or past evidence of dishonesty, that will be coming in, too.
But, sometimes, there are good reasons to let a defendant testify. Domestic cases involving harassment or threats are frequently “he said/she said” and juries want to hear both sides. If it’s the sort of fight where both people were equally hostile - and if you know that your client has no prior criminal history, is reasonably intelligent and sufficiently docile - you are probably wise to suggest that he testify.
Also, if the crime is one of intent (i.e. a business dispute, where the issue is not whether the Defendant took the money, but whether he did it in good faith, or whether it was his intent to steal), letting your client explain their motives might be helpful. (Once again, though, as a defense lawyer you have to be really concerned about the existence of any incriminating details - perhaps an email that essentially confirms that the client knew he was acting improperly - which will become the focus of cross-examination, along with whether the client is sophisticated enough to understand potentially tricky questions, whether the client is at risk of just getting pissed off any losing their shit on the stand, and whether the client has a history of crime or dishonesty that is going to show that he can’t be trusted).