Here is what is going on. At the end of the month the "Bush tax cuts" expire. While most of the cuts go to people at the high end of the income ladder, there are some cuts for people making less than $250,000 that will also expire. The President and Democrats as asking Republicans to go ahead and approve this, since they say they agree. But Republicans are trying to keep the Bush tax cuts for their wealthy backers and are willing to prevent extending these tax cuts for middle in order to do it. In other words, they are holding the middle class hostage. A "discharge petition" can force this bill to come up for a public vote.
The Hill sums it up succinctly, in House Dems try to force vote on extending middle-class tax rates: "House Minority Leader Nancy Pelosi (D-Calif.) on Friday announced Democrats would circulate a discharge petition to force a House vote to extend current tax rates only on annual household income below $250,000. The Senate has already approved the extension, but Republicans are objecting because they want all of the Bush-era rates to be extended, including on income above $250,000."
Sabrina Siddiqui at the Huffington Post reports: "Pelosi called on House Republican leadership to bring that legislation to the floor next week and threatened that if they do not schedule a vote on the Senate bill, Democrats will file what's known as a discharge petition on Tuesday to force a vote on the measure in her chamber. If Democrats successfully obtain 218 signatures on the discharge petition, it would automatically force the middle income tax cut bill to the floor for a vote.
"We believe that not [bringing the Senate bill to the floor] would be holding middle income tax cuts hostage to tax cuts for the rich," Pelosi said. "Tax cuts for the rich which do not create jobs, just increase the deficit, heaping mountains of debt onto future generations."