In Germany’s federal election last month, the Liberals (FDP) more than doubled their vote share to 10.7 percent. Post-election analysis has focused primarily on the losses of Chancellor Angela Merkel’s Christian Democratic Union (CDU) and the rise of the anti-immigration, new national conservative party, Alternative für Deutschland (AfD).

There’s another story here — the CDU actually lost more voters to the Liberals than the AfD, and the FDP was also a clear winner in this election. What does this tell us about shifting party loyalties, and what happens now? Our research gives some clues.

The Social Democratic Party (SPD), which had its worst electoral result in postwar German history, quickly announced that it would not be part of another Grand Coalition with CDU. SPD instead will look to regroup as the leader of the opposition in the next legislative term.

This leaves what may shape up to be a “Jamaica” coalition (named after the colors of the Jamaican flag and the potential coalition partners) between the CDU/CSU (black), FDP (yellow) and the Greens. This would be the first such coalition on the national level.

Political changes that would facilitate the coalition are already fueling speculation about the consequences for European politics. Soon after the election, the CDU’s powerful Minister of Finance Wolfgang Schäuble stepped down to run for the office of president of the parliament. This frees up an important ministerial portfolio for Merkel to use in coalition negotiations — and it’s a position the FDP wants.

The FDP will probably resist pushes for flexibility in the European Union’s Stability and Growth Pact, oppose a large euro zone budget, and maintain Germany’s hawkish position on the question of debt relief for Greece. These are also areas where a renewed Franco-German partnership in the E.U. in the wake of President Emmanuel Macron’s emergence in France will be tested.

This is a remarkable turnaround for the FDP

How is it possible that the Liberals regained this much political power after they failed to clear the 2013 Bundestag election threshold — 5 percent of the vote — to gain access to Germany’s primary legislative chamber?

Historically, the FDP cleared this threshold by comfortable margins and regularly participated as a junior partner in governing coalitions, which are the norm for Germany. In 2013, however, the party received only 4.8 percent of the vote and thus got no Bundestag seats, for the first time in the postwar era. Media reports suggest that the lack of a popular leader was a major reason for the party’s unusually poor performance in 2013.

Many analysts suggest that the popularity of the current party leader Christian Lindner is a key factor in the FDP regaining parliamentary representation in this most recent election. According to a recent poll, Lindner ranks second (behind Merkel) with 62 percent of voters thinking of him as “likable.”

Yes, popularity matters

Here’s what we think is going on. Our research supports the idea that the relative popularity of the party leader helps explain a party’s electoral success. In a new study, we investigated European party members’ voting behavior. We analyzed party membership defection using data from the European Social Survey (2002-2010), the Swedish Party Membership survey and the British Election Study Panel.

More specifically, we tracked defection through the self-reported vote choice in the 2014 legislative election of over 10,000 party members in Sweden and did the same for 2,735 British party members included in two waves of the 2015 BES online panel survey around the time of the 2015 election.

In European democracies, the members of political parties can make important contributions to a party’s electoral success. Party strategists and political scientists alike regularly assume that party members are loyal voters, and thus make a direct contribution to their party’s electoral result. For example, some observers argue that the members of the British Labour Party were in large part responsible for Labour’s surprisingly strong showing in the 2017 general election.

In one way, this should not surprise us. After all, party members have chosen formally to align themselves with that party, and are voting for their own “team.” They are ideologically committed to the party’s goals of securing office and influencing public policy. They also choose to advance these goals with their own time, labor and money.

It’s only logical that party members would also further support these goals with their vote choice on Election Day. This indicates that despite widespread membership decline in Europe, political parties have every reason to count on their membership base as a rather safe reservoir of votes.

We took a closer look at the viability of this assumption using different sets of European survey data. We found that, on average, about 8 percent of party members reported voting for a different party than their own — only slightly lower than the defection rate for self-identified party supporters.

What makes party members defect?

We found that being very dissatisfied with the party leader bumped the probability of voter defection by 11 to 35 percent. This confirms that the politician at the top of the ticket is important to retain even the most loyal of party voters, and suggests that personnel differences are enough to drive a wedge between a party and its most committed voters.

For smaller parties like the FDP, even low levels of defection can make the difference between clearing the parliamentary threshold and receiving no legislative seats, as the 2013 election results showed. And while we would not go so far as to claim that defecting members made the difference for the FDP in the 2013 and 2017 elections, party members are known to be a mobilizer of votes and serve as an important voice for the party in the electorate.

Given the potential importance of parties and party loyalty for electoral success, our work suggests that political parties can’t take the support of their members for granted — and should be attentive to issues that might bring about dissatisfaction among this important voting group. Additional attention in the selection of party leaders and other moves to enhance intraparty democracy are some of the primary recommendations that flow from our research.

Jonathan Polk is an associate professor of political science at the University of Gothenburg, where he conducts research on political parties in Europe and political participation. He tweets at @jon_polk.

Ann-Kristin Kölln is an associate professor of political science at Aarhus University. Her research interests include European political parties, public opinion and survey methodology.