Alternatively, she said, Florida and Michigan should hold new primary elections, probably in early June after the remaining primaries are completed and perhaps by statewide mail-in ballot. She and her advisers believe she would do well in any revote and gain a major boost in delegates and momentum with two season-ending victories.

Mr. Obama, speaking on MSNBC Tuesday evening, ruled out any solution that accepts the January results. “We were told that Michigan and Florida wouldn’t count, and so we said we wouldn’t campaign there,” Mr. Obama said. “Senator Clinton said the same thing, that they wouldn’t count. Now her campaign is suggesting that they should.”

Mrs. Clinton said last October that the Michigan primary was meaningless, but she left her name on the ballot. Mr. Obama and the other major Democratic candidates removed their names from the ballot in a gesture of good faith to early-voting states whose primaries were officially allowed by the Democratic Party. Neither candidate campaigned in Michigan; Mrs. Clinton won with 55 percent of the vote over 40 percent for “uncommitted”

Mr. Obama’s did not spell out his Plan B, but he said that any revote would be problematic, particularly if conducted by mail in Michigan and Florida, two states that have never conducted a mail-in election. He said he would like to see the Michigan and Florida delegations seated in an “equitable” way, without spelling out what that would mean.

Obama advisers declined on Wednesday to say what a fair distribution of delegates would look like but have floated a plan to apportion the delegates 50-50, wiping out any advantage that Mrs. Clinton might have gained from the votes in January and essentially making the two states meaningless in the nomination fight.