WASHINGTON — For months, even in the face of an avalanche of bad news for Republicans, Democratic ambitions for capturing Congress have collided with an electoral map created to protect Republicans from ouster.

Despite polls showing rising support for Democrats and scorn for Republicans, analysts have said Democratic hopes for big gains remain remote, because so few seats are in contention.

That appears to be changing.

Over the past week, a handful of once-safe Republican congressional seats have come into play, and other Republican incumbents are facing increasingly stiff re-election battles, according to analysts, pollsters and officials in both parties. The change amounts to a slight but significant shift in the playing field, and a potentially pivotal change in the dynamics of this midterm election.

In a Republican primary in Pennsylvania on Tuesday, Rep. Don Sherwood drew 56 percent of the vote against a little-known challenger, a display of weakness in a race that both parties now see as being in play.

The Republican Congressional Campaign Committee has spent at least $1.9 million in the June 6 special election to replace Rep. Randy Cunningham, who was forced out in an ethics investigation, in a California district that should be a cinch for Republicans.

On Friday, President Bush flew to the congressional districts of two Republicans who had once seemed heading for easy re-election — Thelma Drake in Virginia and Geoff Davis in Kentucky — to help them raise money. A White House adviser said the decision to send Bush reflected concern about the challenges the first-term incumbents face.

The nonpartisan Cook Political Report, which tracks congressional races, increased the number of Republican seats viewed as competitive on Friday to 36 from 24, said Amy Walter, an analyst there. Democrats seem to be in increasingly good shape to pick up seats in bands of districts across Pennsylvania, Connecticut and New York, as well as districts throughout Ohio, North Carolina, Arizona, New Mexico, California and Florida. Democrats need to pick up 15 Republican seats to take control of the House.

"The playing field is certainly expanding," Walter said. "Clearly what we are seeing is that the political environment is taking a toll and dragging down Republican incumbents; it's dragging down their polling numbers. The question is, What will the environment be in November?"