Editor’s Note: Julian Zelizer is a professor of history and public affairs at Princeton University and the author, with Kevin Kruse, of the new book “Fault Lines: A History of the United States Since 1974.” The opinions expressed in this commentary are his own. View more opinion at CNN.

CNN  — 

The arrest of Roger Stone in the early morning hours Friday is another devastating political blow to President Donald Trump. It gives more ammunition to House Democrats to accelerate their own investigation plans, and it will certainly generate more fear among Republicans, even those who continue to stand by the President.

Robert Mueller’s indictment of Stone over alleged coordination with Trump officials about stolen emails from WikiLeaks brings the number of Trump associates charged in the special counsel’s investigation to six. Many of the indictments have revolved around the ways in which various advisers lied about their connections with Russians during the election as well as their role in trying to obstruct a legitimate investigation into what happened.

White House press secretary Sarah Sanders immediately tried to distance the administration from the news in an appearance Friday on CNN’s “New Day.” She tried the line, “My first reaction is simple: This has nothing to do with the President, and certainly nothing to do with the White House,” but the indictment provides more evidence of the highly suspect activities in Trump’s orbit during the 2016 campaign and ever since. Stone’s indictment, coupled with dramatic footage of his arrest by the FBI, raises more questions about how far the Trump campaign was willing to go to win the election and the efforts it has undertaken to cover up its actions.

The fact that Stone was in Trump’s orbit for so long is problematic in itself. For more than four decades, Stone has been part of the underside of Republican politics. Since his work on behalf of President Richard Nixon, when Stone really pioneered the art of “dirty trick politics,” which used disinformation and character assassination as the primary means toward victory, he has been willing to sell his talents to whichever candidate was willing to listen to him.

Many Republicans have turned to him for his services since the 1970s, including Ronald Reagan in his 1980 campaign. While Republicans like to talk about the free market ideology and tough national security ideas they have brought to the table as the reason for their political success over the decades, a large number of the top players in the party have also been willing to go really low when it came time to win campaigns.

During his work for Nixon and Reagan, as well as during his time in the 1980s co-heading a major lobbying firm with Paul Manafort that worked with notorious overseas authoritarian leaders, Stone always was willing to get as close to the line of illegal as possible in pursuit of victory and access to power. A recent film about Stone captured him sharing some of the famous sayings that reflect his outlook: “It is better to be infamous than never to be famous at all” and “one man’s dirty trick is another man’s civil political action.”

Trump and Stone have a relationship that goes back many decades. Stone was part of the discussions that resulted in Trump running for the presidency.

What impact will Stone’s indictment and arrest have? Legally, the answer is that we just don’t know. The ways in which his story and actions fit into the broader findings that Mueller is putting together remain to be seen.

Politically, there’s no doubt that this is a damaging development. It comes at a time when the Republican firewall that has protected Trump has shown some slight but serious cracks as a result of the disastrous government shutdown. A number of Republicans voted with the Democratic spending bill that the Senate ended up rejecting. Sandwiched between the Democratic takeover of the House in 2018 and the fears about what will take place in the 2020 election, Republicans are sweating it out every day about what political costs they will pay from ignoring the scandal for so long.

From the beginning, the outcome of the entire investigation has been as much a political as a legal issue, and the dynamics fundamentally changed when Democrats took control of the House. If Trump had any illusions about how firm House Speaker Nancy Pelosi would be in dealing with the chaos caused by this administration, those should have been quickly dispelled during the shutdown crisis. Several committees are now gearing up to launch different kinds of investigations.

Stone’s indictment, which gets the investigation so close to the original question of what kind of coordination may have transpired between Trump campaign and Russian officials, is a huge splash of cold water on the President’s defenders, who say there is nothing there.

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    Each week seems to bring further information about the wrongdoing of the people whom Trump surrounded himself with, the people whom he trusted to run his campaign and some who have worked with him ever since.