A series of recent state polls around the country confirm earlier forecasts about the uphill challenge faced this year by Republicans in trying to hold their ground in the Senate, or even staving off new Democratic gains.

Republicans are defending 23 seats this year with five of them open due to retirements while Democrats are defending only 12, all filled by incumbents. That was the basis of a CQ Politics forecast in April that Democrats were well-positioned to add two and possibly as many as eight seats to their 51 to 49 working majority (including independents Joseph I. Lieberman of Connecticut and Bernard Sanders of Vermont).

Surveys in the last few weeks found Republican incumbents teetering near or below 50 percent in the polls, or actually running behind Democratic challengers. In two states where seats were left open by retiring Republicans, the Democratic candidates were running ahead.