Republican presidential candidate Donald Trump speaks before a crowd of over 3,500 Saturday, July 11, 2015, in Phoenix. (AP Photo/Ross D. Franklin)

Reince Priebus, chairman of the Republican National Committee (RNC), has asked Republican presidential candidate Donald Trump to “tone down” his rhetoric on Mexican (illegal) immigrants.

Trump has been under fire since his campaign announcement that these type of immigrants are “bringing drugs” and “crime” into the United States, and are “rapists” (he did also add that “some, I assume, are good people”). Trump’s comments have resulted in a major backlash, with a growing number of corporations – like NBC Universal, NASCAR, ESPN, and Macy’s – severing ties to him.

The Republican Party cannot break up with Trump as easily, however. Many Republicans now fear that Trump’s unfiltered comments will irreparably damage not only his own reputation but also that of the Republican Party brand – that is, the set of issues and policy positions that voters associate with the Republican Party.

[Listening to Donald Trump swear and talk politics on his private plane]

Why is Priebus, as chairman of the RNC, the person in the party qualified to call Trump to order? And what kind of powers does the chairman of the national committee have to make presidential candidates like Trump toe the line?

The answers underscore the complicated situation the Republican Party finds itself in with a candidate like Trump. On the one hand, Priebus is currently the party’s only truly national leader and, as such, is the logical person to try and step in and police his party’s brand. On the other hand, chairmen like Priebus have very few concrete powers to force compliance from the party’s candidates.

The Democratic and Republican national committees, both founded in the middle of the 19th century, were established initially to organize the national conventions every four years. Outside presidential election years, they were dormant. Towards the end of the 19th century, the national committees became more active – organizing the presidential campaigns and providing support in congressional and local elections – as the parties began to rely more on “educational” initiatives to bind voters to them.

Since the early 20th century, both national committees have maintained permanent headquarters in Washington and provide “services” to party members year-round. These services include collecting a substantial amount of campaign funding; in the 2012 election the DNC and RNC spent more than $723 million. Some funds are given directly to candidates, but most are used to help organize campaigns and educate voters on party positions through extensive advertising operations.

Political scientists, like Philip Klinkner and Daniel Galvin, have shown that the national committees are generally more active when their party is not in control of the White House. This is not surprising. When a party has an incumbent president in office, he is seen as the party’s national leader.

However, without a president in office, the chairman of the national committee is the sole political actor who represents the party as a whole. All other office holders – including the party’s leadership in Congress – represent only a limited portion of the country or political system. Cordell Hull – who was chairman of the Democratic National Committee (DNC) from 1921 to 1924 and would later serve as FDR’s secretary of state – noted in his memoirs that “with the Party out of power and in the minority in both Houses of Congress, whoever occupied the office of chairman of the National Committee was in the highest position of Democratic Party leadership in the nation.”

National chairmen during years when their party was out of the White House have frequently used this highest position of leadership to guard the party’s national brand and, if necessary, push the party in their desired policy direction. As a result, chairmen of the national committees in out-years often clash with other members of their party.

During the late 1920s, DNC chairman John Raskob aggressively tried to push the Democratic Party to support a repeal of prohibition – to the frustration of conservative Democrats. Less than a decade later, RNC chairman John D. Hamilton attempted to bring the Republican Party together in a unified response to the New Deal.

During the 1950s and 1960s, chairmen of the DNC and RNC – in opposition to congressional leaders – attempted to move their parties towards more consistently liberal and conservative positions (respectively) on civil rights. More recently, Howard Dean tried to push his Democratic Party to adopt a more centrist position through what he called a “50 state strategy,” which relied on recruiting Democratic candidates who were soft on social issues and therefore would be competitive in more conservative districts. This move to the center was effective in the 2006 and 2008 elections, but also frustrated some liberals in the party.

In all of these cases, national chairmen insisted that they had the right – indeed, the duty – to police the party’s brand, and attempted to constrain party members accordingly. Given this history, Priebus stepping up to protect what he believes the Republican brand should be from Trump’s inflammatory remarks is unsurprising.

But can national chairmen successfully force compliance? In previous eras, chairmen could credibly threaten candidates that a failure to hew to the party brand would result in reduced financial support from the party. But such a threat is decidedly less compelling in a post-Citizens United world, since Super PACs can easily overcome any reductions in support from the party. Additionally, in the particular case of Trump, Priebus faces a billionaire who can self-fund a vanity campaign for a considerable amount of time.

What about the threat that a presidential nominee would need to rely on the national party’s campaign expertise to win in a general election? That too has become less compelling. Presidential candidates have increasingly relied on their own personal organizations, and through modern (social) media can easily connect with voters without the national party. The best recent example of this phenomenon is Barack Obama, who built a massive stand-alone campaign organization that not only allowed him to win two presidential elections without relying on the DNC for campaign support, but which now has been restructured as Organizing For America (OFA) – a new political organization entirely separate from the DNC.

Given the role that the national committee chairman plays as guardian of the party’s brand in out-years, it is entirely customary for Priebus to call out candidates like Trump when they appear out of step. But neither party’s chairman has the power to excommunicate individual members. And they now have fewer tools with which to sanction those who dilute the brand.

This means that Priebus will have to rely on carrots rather than sticks to rein in the party’s more extreme voices. In the case of Trump, Priebus will need to resort to persuasion – by convincing Trump that a coherent Republican Party brand is more important than airing his own views.

So far, there is little indication that Trump has seen the wisdom of this argument. For now, Trump remains a viable option for Republican primary voters. How much damage he will do to the Republican Party brand remains to be seen.

Boris Heersink is a Ph.D. student in the Woodrow Wilson Department of Politics at the University of Virginia and a national fellow at the Miller Center. Jeffery A. Jenkins is Professor in the Woodrow Wilson Department of Politics at the University of Virginia and co-author of “Fighting for the Speakership: The House and the Rise of Party Government.” Much of the material for this blog post is based on Heersink’s dissertation: “Beyond Service: National Party Organizations and Party Brands in American Politics.”