Stan Greenberg, a Democratic pollster who worked for President Bill Clinton in the 1994 midterm elections, said polls he conducted in three districts where Democrats were thought to have modest hopes of winning found incumbents struggling with just 50 percent of the vote against unknown challengers. In a typical year, an incumbent at this point should have an overwhelming lead.

The members in question were Chabot in Ohio, Curt Weldon in Pennsylvania and Mike Ferguson in New Jersey. "These races are competitive five or six months before the election," Greenberg said.

Republican officials said they were confident of holding on to all three seats.

Republicans have an additional problem as a price for being in power for so long, analysts said: Many of the members who suddenly have fights on their hands are accustomed to coasting to victory and may not be prepared for what awaits them in the months ahead.

Republicans say that even in this difficult environment, they see one big advantage: The party is focusing on the challenge early enough in the year to try to overcome it. King of New York said that stood in contrast to 1994, when the Democrats were caught off guard by the late surge led by Newt Gingrich for control of the House.

"They're concerned — which is good," King said of fellow House Republicans. "People make the comparison to '94. I was here in '94. One of the main advantages we have over the Democrats is they never saw it coming — not at all. We see six months out that there is a real problem that we have to face."

Bill McInturff, a Republican pollster, said this would give the party time to do what it did in 1996 to head off a fierce Democratic challenge.

"Raise more money, disqualify the opponent early, and don't let them off the mat," he said. "That's what we'll try to do."

To date, the battle over Congress has been defined by a conflict between two forces. On one hand, there is abundant polling data suggesting that voters are looking for a change after six years of Republican dominance.

The problem for Democrats, however, is that the process by which lawmakers draw congressional boundary lines has had the effect — at least mathematically — of putting a vast majority of districts off-limits to Democrats searching to knock out incumbents, pending the kind of electoral wave Democrats have sought.

But as races across the country suddenly began popping up on lists of contests in play — Rep. Richard W. Pombo of California, Charles H. Taylor of North Carolina, the seat held by Henry J. Hyde, the Illinois Republican who is retiring — some analysts are saying that that wave appears to be coalescing.