Republicans in the House who have refused to sign on to a petition allowing a vote on extending Bush-era tax cuts are being targeted in radio and web advertisements by Democrats, who want to extend the cuts only on households making less than $250,000 per year.

The Democratic Congressional Campaign Committee said Wednesday they would air the ads against 21 House Republicans who have refused to sign the discharge petition advanced by Democrats. The Bush-era cuts expire at the end of the year.

"Tea Party Republicans in Congress are about to send our middle class tax cuts off the fiscal cliff," a narrator says in the radio spots, encouraging listeners to tell their representative: "Don't hold the middle class hostage to get more tax cuts for millionaires."

The Democrat-controlled Senate has passed a bill extending the Bush-era tax cuts on incomes under $250,000, but Republicans who control the House haven't brought the measure for a vote. They have passed a measure themselves extending the tax cuts for all earners.

The discharge petition requires a majority of House members to sign on, meaning some Republicans are necessary in order for the Senate measure to come for a vote. Republicans currently hold a 241-192 majority in the lower chamber.

"House Republicans have a chance to stand with the American people but instead they continue to hold the middle class hostage," DCCC Chairman Steve Israel said.

The issue of the Bush-era cuts is central to negotiations over the year-end fiscal cliff, which combines across-the-board tax hikes and massive spending cuts to reduce the federal deficit. The White House and House Republicans are currently haggling over ways to increase revenue and reduce spending, with major sticking points coming on increasing tax rates for wealthy Americans and specific cuts to entitlement spending.

On Tuesday, a top independent conservative group launched radio spots targeting five Democratic senators in the battle over negotiations to prevent the country from falling off the fiscal cliff.

Crossroads GPS said its ads urge the Democrats to support significant spending cuts as part of any deal. The group said it's spending $240,000 to run the radio commercials. DCCC did not release the size of their radio ad buy.