A federal judge declared a mistrial on Wednesday in the case of six Miami men charged with plotting to blow up the Sears Tower in Chicago as part of an Islamic jihad.

It was the second mistrial in the federal case. Legal analysts called the outcome a significant defeat for the Bush administration, especially its publicizing of terrorism arrests.

''In a lot of these cases, the government has really oversold what it's got,'' said Jenny Martinez, an associate professor of law at Stanford who was involved in the Jose Padilla terrorism case. ''They've held these huge press conferences at the beginning that set up these expectations that the government cannot fulfill.''

The approach, analysts said, often smacked of politics. In this case, when the seven men from the Liberty City area of Miami were arrested a few months before the 2006 elections, Attorney General Alberto R. Gonzales outlined the most sensational evidence at a news conference. He said the men had been taped promising to fight a ''full ground war against the United States.''

The jurors faced far less clear-cut evidence. Testimony showed that a search by the F.B.I. of what it called the group's headquarters did not find guns, explosives or blueprints for an attack.

Jurors also heard defense lawyers emphasize that the defendants made their most aggressive comments in response to questions or comments by a bureau agents posing as operatives of Al Qaeda and offering $50,000 to help the plot.

The most serious charge was conspiring to provide ''material support'' to a terrorist organization. Prosecutors tried to prove how intent the men were on attacking the United States by citing their loyalty oath to Al Qaeda.

The first trial ended in December with an acquittal for one of the seven, Lyglenson Lemorin, and a mistrial for the other six, Narseal Batiste, accused of being the ringleader; Patrick Abraham; Burson Augustine; Rotschild Augustine; Naudimar Herrera; and Stanley G. Phanor.

The second trial followed a similar path. Each side laid out many of the same arguments and again the jury deadlocked.

After 13 days of deliberations, Judge Joan A. Lenard of Federal District Court ordered a mistrial.

A statement from the office of the United States Attorney R. Alexander Acosta said a decision on a retrial would be announced at a hearing next Wednesday.

The jurors did not comment. Bruce J. Winick, a law professor at the University of Miami who followed the case, said the mistrial showed public skepticism of the pre-emptive terrorism-related arrests.

''Politics played too important a role in this prosecution,'' Professor Winick said. ''We should follow our normal prosecution pattern, which is to gather the evidence.''

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