"But a year of government inquiries and blockbuster journalism has made clear that the dossier is unquestionably real news", he wrote.
Michael Cohen, who worked for the Trump Organisation and is now a personal lawyer to the president himself, is named in the dossier.
Mittenthal added: "This is not the first time Trump's personal lawyer has attacked the free press, and we look forward to defending our First Amendment rights in court".
Josh Levy, a lawyer for Fusion, declined comment Wednesday morning.
"It has helped journalists explain to their audience the investigation into Russian influence on the 2016 election", he said. Cohen testified to the House Intelligence Committee behind closed doors on October 24 and publicly to the Senate Intelligence Committee on October 25.
He said that he and Trump "don't talk about" the Russian Federation investigations.
Cohen said he told the panels that he never engaged with, was paid by, or communicated with anyone representing the Russian government, or anyone else, about hacking or interfering with the US election, hacking the Democratic Party, or about creating fake news stories to assist the Trump campaign or to damage the Hillary Clinton campaign. "Why waste time talking about something that's not legitimate", he said.
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Mr Mueller's team has produced two indictments and two guilty pleas by former aides to Mr Trump's campaign.
"We're a year down the road now", Cohen said.
The committee is in the process of scheduling eight witnesses including Federal Bureau of Investigation agent Peter Strzok and Federal Bureau of Investigation lawyer Lisa Page, who exchanged anti-Trump text messages during an affair and previously worked on the special counsel's Russian Federation probe; Federal Bureau of Investigation general counsel James Baker, who was reassigned; Federal Bureau of Investigation head of counterintelligence Bill Priestap, whom ex-FBI boss James Comey testified made the decision not to brief Congress about the Russian Federation case during last year's election; Federal Bureau of Investigation special agent and chief of staff James Rybicki; and Federal Bureau of Investigation congressional liaison Greg Brower.
In his op-ed, Smith asserted that the dossier provided critical context for the American public to understand the actions of the various political players in the metastasizing investigation of Trump and his campaign's ties to Russian Federation.
Cohen is also reportedly suing Fusion GPS - the intelligence firm that commissioned the research into the alleged ties between Trump and Russian Federation - and its cofounder, Glenn Simpson, Bloomberg reported. Australian officials then passed the information to their American counterparts, the Times reported.