Americans’ opinion of the Democratic and Republican parties is now at its lowest level in decades, according to a new CBS News poll.

Favorable views of the Democratic Party dropped 20 points in the past year to 37 percent, according to the poll, conducted May 20 – 24. Last month, the party’s favorability rating stood at 42 percent.

Fifty-four percent of Americans have a negative view of the Democrats, the poll shows.

This does not mean opinions about the Republican Party have improved as a result. It fares even worse. Only 33 percent of people hold a favorable view of the GOP, with 55 percent holding an unfavorable opinion and 12 percent undecided.

Assuming almost no one holds a favorable view of both major parties, this means that 30 percent of the country does not have a positive opinion of either party. That is a serious level of discontent with voters’ choices in our two-party system.

Normally, when the majority is upset with the only two choices, you would expect a third choice to come along. But our system of single-member districts, plurality-winner elections and tough ballot-access laws creates a huge systematic barrier preventing third parties from reaching the critical mass needed to become viable. Instead, the disappointment with the establishment parties has recently been taking the form of several successful insurgent primary challengers.

This disgust with both parties is playing an important role in the Florida Senate race, where Charlie Crist is polling well as an independent against Republican Marco Rubio and Democrat Kendrick Meek. Crist leads in a recent poll with, incidentally, 30 percent of the vote.