Voter preference for the two party system seems in constant decline and is confirmed by recent polling. Ballot Access News by Richard Winger had the following summary:

Every few months, Gallup asks voters if they identify as Democrats, Republicans, or something else. For respondents who say they are something else, they are then asked if they lean toward either the Republican Party or the Democratic Party.

During the last few months, the percentage of voters who identify as Democrats, or who say they lean toward the Democratic Party, has remained the same, at 45%. The percentage of voters who identify as Republicans, or who say they lean toward the Republican Party, has dropped from 40% to 38%. See the results here. The poll was taken during May but only released on June 6. Thanks to Political Wire for the link.

According to the Gallup report:

The growing Democratic advantage in recent months is mostly attributable to a decline in Republican affiliation rather than an increase in Democratic affiliation. Since November, the percentage of Republicans and Republican leaners has fallen four percentage points, while there has been a one-point rise in Democratic identification or leaning. The Republican decline has been offset mostly by a three-point increase in the percentage of Americans with no party preference or leaning.

President Donald Trump’s unpopularity is likely a factor in the Republican Party’s falling fortunes. The president’s job approval rating generally has held near 40% since February — well below average for presidents historically, and especially for those in their first few months in office.