Most states send senators of the same party to Congress

Only six states now have U.S. senators of different parties – the smallest number of split delegations since Americans started directly electing their senators more than a century ago, according to a new Pew Research Center analysis.

The number of split Senate delegations has ratcheted downward since peaking at 27 in 1979-80. There were just nine split Senate delegations in the recently concluded 116th Congress, which tied the prior record low.

This post examines the long-term decline of politically divided delegations in the U.S. Senate. The analysis begins with the 64th Congress of 1915-17, the first full session following the ratification of the 17th Amendment, which provided for the direct election of senators. (Before the amendment was ratified in 1913, senators were chosen by state legislatures, though a few states had opted to give voters some kind of say. The dynamics of the chamber under that system were so different as to not be readily comparable to the elected Senate.)

Our main source for the composition of the Senate and party affiliation of individual senators was the online Biographical Directory of the United States Congress, supplemented by contemporaneous media reports.

We tabulated the party affiliations of each sitting senator at the start of each Congress, and also noted changes in membership and party affiliation during each Congress’ term. Independents and third-party senators are classified with the major party they caucus with, if any.

This analysis examines every Senate since the general election of 1914, the first one after ratification of the 17th Amendment, which provided for the direct election of senators.

From the beginning of our analysis until the late 1960s and early 1970s, the number of split Senate delegations generally oscillated between 10 and 20. The count fell to nine in the closely divided 84th Congress (1955-56), in which Democrats held a 49-to-47 majority thanks to independent-turned-Democrat Wayne Morse of Oregon.

U.S. Senate now has fewest split delegations in more than a century

Politically divided delegations became more common in the 1960s and ’70s, as decades-old patterns of state-level party dominance began to break down. By the 96th Congress of 1979-80, more than half the states (27) had split delegations; from 1973 through 1994, there were never fewer than 20 split delegations in the Senate.

Since then, however, the trend has been toward more and more single-party delegations, as partisanship has grown and many long-serving senators died or retired. It wasn’t that long ago that states like Arkansas, Louisiana and Nebraska could send two Democratic senators to Washington, while Republicans could and did win in states like New York, Illinois and Oregon.

In another reflection of the partisan polarization that has reshaped American politics over the past few decades, states’ current Senate delegations are remarkably aligned with their presidential preferences.

In the current Congress, all 22 states with two Democratic senators went for Democratic nominee Joe Biden in the November election, and all 22 states with two Republican senators went for GOP incumbent Donald Trump. Three of the states with split Senate delegations – Montana, Ohio and West Virginia – chose Trump, while the other two (Pennsylvania and Wisconsin) opted for Biden. (Two states, Maine and Nebraska, allocate their electoral votes by congressional district. Maine, which has a split Senate delegation, gave three of its four electoral votes to Biden and one to Trump. Nebraska, which has two Republican senators, gave four of its five electoral votes to Trump and one to Biden.)

The partisan divisions weren’t always as clear-cut as they are today. Consider the 87th Congress, elected in 1960 and seated in 1961. Of the 25 unified Democratic Senate delegations, only 13 represented states that gave their electoral votes to John F. Kennedy the year before, while 10 were from states that had chosen Richard Nixon. (Mississippi chose an anti-Kennedy “unpledged Democratic” slate of electors, and Alabama chose a “mixed” slate of Kennedy and unpledged electors.) Of the 10 states represented by two Republican senators, three chose Kennedy for president instead of Nixon. The 15 states with split delegations were split nine-to-six for Nixon.

Political scientists have explored the question of why a state’s voters, whose party preferences and turnout behavior presumably don’t change that much from one election cycle to another, would elect senators of different parties. One school of thought is that some voters deliberately seek to balance their state’s delegation, though other researchers have not found support for that idea. Other researchers have focused on diverse electorates as predictors of split delegations, argued for candidate-specific factors (such as financing, campaigning skill and the presence or absence of scandal), or tied the ebb and flow of split delegations to broader partisan realignments.

Whatever the explanation, several states have sent pairs of senators to Washington who were so ideologically disparate that their votes all but canceled each other out. Past examples include Minnesota’s Paul Wellstone and Rod Grams, North Carolina’s John Edwards and Jesse Helms, and California’s Alan Cranston and S.I. Hayakawa (each Democrat and Republican, respectively). A few years back, the Capitol Hill publication Roll Call took a look at some of the Senate’s “odd couples,” awarding the top spot to Wisconsin’s Ron Johnson, R, and Tammy Baldwin, D.

Having fewer split delegations in the Senate likely diminishes opportunities for bipartisan cooperation. Senators from different parties may disagree on everything else but can sometimes work together on matters of special interest to their state. In 2006, for example, Florida Sens. Bill Nelson, D, and Mel Martinez, R, teamed up on a bill to prohibit offshore oil and gas drilling off the state’s Gulf Coast. Last year, Montana Sens. Steve Daines, R, and Jon Tester, D, collaborated on legislation to settle a decades-old water dispute between Indigenous tribes and the state and federal governments.

Drew DeSilver  is a senior writer at Pew Research Center.