A few months ago, many progressive Democrats were elated when Al Franken was finally declared the winner of the disputed Senate seat from Minnesota. Franken’s victory supposedly gave Democrats a “filibuster-proof” 60-vote majority so Republicans would no longer be able to block progressive legislation or extract major concessions in exchange for their support. But the progressives’ joy was short-lived. Despite the largest Democratic majority in decades, the Senate still has not passed a health care reform bill and it is not clear when, or even if, it will. If the Senate does pass a bill it will almost certainly provide less generous benefits than the one passed by the House of Representatives. Climate change legislation, which passed the House months ago, remains stalled in the upper chamber. And only a handful of President Barack Obama’s judicial nominees have been confirmed.

The Senate remains without question the greatest obstacle to the Obama Administration’s domestic policy agenda and the hopes of progressives for changing the nation’s priorities. The main problem, of course, is the filibuster rule which allows a minority of senators to block the will of the majority. For all practical purposes, it takes 60 votes, not a simple majority, to pass legislation.

The increasing use of the filibuster in the Senate is a direct result of one of the most important developments in American politics in recent years — the rise of partisan polarization. As the ideological differences between Democrats and Republicans have increased and the number of moderates in the Senate has decreased, the possibility of building bipartisan coalitions has drastically declined. Instead of working with members of the majority party to craft legislation that can pass the chamber, minority party members find it much easier to use the filibuster to prevent the majority party from passing any legislation.

With no prospect of winning more than one or two Republican votes, Democratic leaders need to maintain perfect or near-perfect unity in their caucus in order to win a cloture vote. That’s not an easy task, and it’s likely to get even more difficult after the 2010 and 2012 elections. The current 60-seat Democratic majority is by far the largest for either party in the past two decades. While Democrats have a good chance of retaining control of the Senate in the next two election cycles, their majority is almost certain to be reduced.

In 2010 Republicans will be defending 19 of the 38 seats that are up for election, so their opportunities for gains will be limited. In 2012, however, Republicans will have a much better chance to recoup some of the losses that they suffered in the 2006 and 2008 elections because Democrats will have to defend 24 of the 33 seats that will be up for election.

The results of the 2010 and 2012 Senate elections will depend on the national political climate when those elections take place. In the long run, however, Democrats will probably find it very difficult to maintain anything close to a 60-seat majority in the Senate.

Some of the greater vulnerability of Senate Democrats is a direct result of the Senate’s small-state bias. Because every state has two senators, regardless of population, states with small populations are vastly over-represented in the Senate. A senator from California, the most populous state, represents more than 70 times as many people as a senator from Wyoming, the least populous state.

The over-representation of small states in the Senate clearly favors the Republican Party. That’s because Democratic voters tend to be concentrated in large metropolitan areas which are found mainly in the most populous states. In contrast, Republican support is generally greatest in the nation’s small towns and rural areas. As a result, Republicans have carried 12 of the 20 least populous states in all three presidential elections since 2000 while Democrats have only carried five of these states in all three elections.

A reduced Democratic majority will make it almost impossible to invoke cloture. This leaves progressive Democrats with two options: try to build bipartisan coalitions or change the Senate’s rules. Bipartisanship is very popular with many Washington political insiders. However, given the deep ideological divide that separates the two parties, bipartisanship is simply not a viable option in today’s Senate. In a Senate with a narrow Democratic majority, the swing vote on cloture would not be moderate Maine Sen. Olympia Snowe. It would be someone like conservative Kansas Sen. Sam Brownback. In order to gain enough Republican votes to invoke cloture, progressive Democrats would have to abandon many of their key policy commitments.

A reduced Democratic majority would leave only one viable option for progressives to save their policy agenda: change the Senate’s rules to end the filibuster. Short of a constitutional amendment, nothing can be done about the small state, Republican bias of the Senate. But the Senate’s rules can be changed by a simple majority vote. All it takes is the political will to drag the Senate kicking and screaming into the 21st century.