despite a series of trials that have put Mr. Obama on the defensive and illustrated the burdens he might carry in a fall campaign, the Obama campaign is rolling along, leaving Mrs. Clinton with dwindling options.

Mr. Obama continues to pick up the support of superdelegates — elected Democrats and party leaders — at a quicker pace than Mrs. Clinton.

Even after Mrs. Clinton’s victory in Pennsylvania, Mr. Obama has held on to a solid lead in pledged delegates

the uncommitted superdelegates, who are likely to hold the balance of power — still seem to view their decision the way the Obama campaign would like them to see it.

“The delegate math is not going to change significantly,” Ms. McCaskill said

I don’t see the people who are for Obama wavering one bit.”