The Senate Democratic Party leadership. (Melina Mara/WashingtonPost)

House Republicans are planning to raise country's borrowing limit for three months, with a promise to raise it again if Senate Democrats pass a budget. Sen. Chuck Schumer (D-N.Y.) promised Sunday that it would happen. If it does, it would be the first time since 2009 that Senate Democrats have passed a budget.

Why is that?  Here's a (relatively) simple explanation.

* A budget isn't necessary. Budget resolutions don't have the force of law. Appropriations bills are where spending is allocated. While the budget provides a long-term framework for spending and revenue, lawmakers don't actually have to pass one. While the Senate is legally required to pass a budget, there is no penalty for not doing so.

"The fact is that you don't need a budget," Rep. Steny Hoyer (D-Md.) said last year. "We can adopt appropriation bills and we can adopt authorization policies without a budget."

Since August of 2011, Democratic senators have contended that the Budget Control Act that ended the last debt ceiling crisis set spending caps for the next several years, eliminating the need for a separate budget. Schumer said Sunday that Democrats would have passed a budget this year with or without the House GOP threat because the Budget Control Act's specific spending targets end in 2013. Democrats say that in 2010, lawmakers shifted their attention from a Democratic budget, which did get out of the Senate Budget Committee, to the Simpson-Bowles plan, which came out at the end of the year.

* Democrats don't want the blame. In the Senate, proposing a budget gives Republicans an opportunity to attach amendments that would put political pressure on moderate and conservative Democrats, many of whom already have an eye on their re-election races in 2014. Budget deal-making has always been difficult and budgets almost never come in on time; increasing partisanship has made it even worse. By not introducing a budget, Democrats can keep their names off plans that detail high spending and high deficits. Meanwhile, they can attack House Republicans for their controversial budget plans. (See Ryan, Paul.) That was an especially attractive option for Democrats in the election years of 2010 and 2012. Since the 2010 election, Senate Democratic budgets have had little chance of survival in the House -- the unpalatable legislation would be doomed anyway. (A Senate budget resolution cannot be filibustered but without House agreement, there's no consensus for the coming fiscal year.)

* Democrats couldn't decide on one. Former Senate Budget Committee Chairman Kent Conrad (D-N.D.) repeatedly drafted budgets and attempted to bring them to the floor. But pressure from liberal Democrats on one side and conservative Democrats on the other made it a struggle. In 2011, Conrad proposed a budget that many Democrats attacked as going too far to the right. In May of that year, Conrad announced that Democrats were "very close to an agreement" after some concessions to liberals, but that he would defer to the high-level bipartisan negotiations over the debt ceiling. Democrats did decide on a blueprint in July but did not put it up for a vote. In 2012 Conrad worked on a modified version of Simpson-Bowles but with little hope of a vote. Now, he is gone -- having retired at the end of the 113th Congress.

Democrats have a new incentive to pass a budget -- it's a chance to reopen a debate over tax increases that Republicans insist ended with the "fiscal cliff" deal. A budget cannot be filibustered, and it opens the way to deficit reduction via reconciliation. If language is included in the budget resolution that directs a congressional committee to meet certain spending or tax targets, then the resulting bill is also protected from filibuster.

“There’s going to have to be some spending cuts, and those will be negotiated,” Schumer told the Post. “But doing a budget is the best way for us to get revenues.”