With negotiators nearing an accord on permanent tax breaks for businesses worth $440 billion over 10 years, President Obama rallied Democratic opposition on Tuesday and promised a veto.

“The president would veto the proposed deal because it would provide permanent tax breaks to help well-connected corporations while neglecting working families,” said Jennifer Friedman, a White House spokeswoman.

The deal, negotiated by House Republicans and aides to Senator Harry Reid of Nevada, the outgoing majority leader, showed how much power has shifted since the Republican election victories this month. The negotiations fractured Democrats, and separated the Obama administration from Mr. Reid.

But Mr. Obama’s threat showed that he could still wield his authority as well. White House officials said the package, intended to avoid letting a host of politically popular tax breaks expire at the end of the year, is too heavily tilted toward corporations and will have deep repercussions for budget and tax negotiations far into the future.