Republicans are trying to undermine Vice President Joe Biden's effectiveness as the Obama campaign's chief surrogate and liaison to white, working-class voters and seniors - two influential groups of voters being courted aggressively by both parties.

The relentless attacks are aimed at portraying President Barack Obama's running mate as a governing liability.

Republicans also hope the attacks raise the stature of GOP vice presidential candidate Paul Ryan, who will debate Biden next month, and will score points in closely contested states such as Ohio, Florida and New Hampshire.

Yet Biden himself also gives as good as he gets.

He often is the loudest voice in the campaign's criticism of the Republican nominee, Mitt Romney.