WASHINGTON -- Republicans are on offense in scores of House and Senate races as persistent economic woes and lukewarm support for President Barack Obama continue to weaken Democrats' hold on Congress.

The president and his party are determined to minimize the losses six months before the November elections. But Democrats privately acknowledge the economy and support for Obama must improve before then to avoid the defeats that could cost them control of the House and possibly the Senate.

Primaries in Indiana, Ohio and North Carolina on Tuesday kick off an intense eight weeks of contested elections. There also are two special House elections to fill vacant Democratic-held seats in Pennsylvania and Hawaii. The outcome could be a clear indicator of the political mood.

"I need your help once more," Obama says in a video message to backers, a plea that underscores the troubles for Democrats. "This year, the stakes are higher than ever," the president adds, warning that Republicans would "undo all that we have accomplished."

Obama and his party must defend dozens of seats in the 80 or so House races that are competitive; they include some districts that Democrats have held for decades. The party also faces serious Senate challenges in at least nine states, including Nevada, where Majority Leader Harry Reid trails in the polls. Democratic seats in Illinois and Delaware, once held by Obama and Vice President Joe Biden, respectively, are in jeopardy, too.

While Republican prospects are looking up, infighting between moderates and angry conservatives might dash the party's hopes. Gov. Charlie Crist's decision to run as an independent set up a three-way Senate race in Florida that could cost the GOP the seat. A similar party battle in Kentucky, which holds its primary May 18, is creating headaches for Republicans.

At this point, analysts for both parties say Republicans probably will pick up as many as three dozen House seats and possibly the 40 needed for control. The GOP is expected to win a few Senate seats, though the 10 necessary to take control is considered a long shot.