“It is not very satisfying for the jury if you hand them a guilty plea and then ask for life,” said Rosanna Cavallaro, a law professor at Suffolk University. “It leaves the jury feeling like, ‘The only thing I’ve done as a juror is give you a break.’”

The government still has to prove its case beyond a reasonable doubt. And it seems to be trying to do so as zealously as if Ms. Clarke had never made her admission. Government witnesses have not scrimped on the details of the carnage at the marathon finish line, accounts that have moved many in the courtroom to tears.

In continuing its full case, the prosecution intends not only to prove guilt, but to justify why it is seeking the death penalty.

“In a run-of-the-mill criminal case, judges will often rush the government along, saying, ‘Do you really need this witness?’ ” said Jeremy M. Sternberg, a former federal prosecutor in Boston and now a partner in the Boston office of the law firm Holland & Knight. “But that won’t happen here. There’s too much at stake.”

Ms. Clarke is trying to use this phase of the trial to her advantage, planting seeds now that she hopes will bloom in the second phase, when she can explain her narrative more fully as the jury contemplates the sentence.

On Monday, for example, defense lawyers pressed one official to establish that Mr. Tsarnaev’s brother was a few steps ahead of him as they walked down Boylston Street. This line of questioning might have suggested what little evidence the defense has to make its case, but at the same time, it helped reinforce the defense’s theme that Tamerlan Tsarnaev was the leader.

On Tuesday, after the government suggested that posts from Dzhokhar Tsarnaev’s Twitter account revealed him to be a jihadist, the defense showed that some of them were in fact lyrics from rap songs and from a show on Comedy Central — more reflective of Mr. Tsarnaev’s college stoner culture than of a holy war.