Dec 28 (Reuters) - A U.S. judge on Tuesday allowed a criminal case relating to the deadly Jan. 6 Capitol attack to move forward, declining to dismiss charges against four members of the far-right Proud Boys group. In a written ruling, U.S. District Judge Timothy Kelly rejected arguments by the four Proud Boys defendants that obstruction charges should be thrown out. The defendants — Ethan Nordean, Joseph Biggs, Zachary Rehl, and Charles Donohoe — have been charged with violating a federal law that makes it a felony to obstruct an official government proceeding, among other charges. That obstruction charge has been used by prosecutors in more than 230 of the 700 criminal cases against participants in the assault. Four people died on the day of the riot by supporters of then President Donald Trump and one Capitol police officer died the next day of injuries sustained while defending Congress. Hundreds of police were injured during the several-hour onslaught and four officers who guarded the Capitol have since taken their own lives. The Proud Boys defendants said the obstruction law is unconstitutional because it is vaguely written and could chill free speech rights. Kelly, who sits in the District of Columbia, rejected that argument. "No matter Defendants political motivations or any political message they wished to express, this alleged conduct is simply not protected by the First Amendment," Kelly said in his ruling. "Defendants are not, as they argue, charged with anything like burning flags, wearing black armbands, or participating in mere sit-ins or protests." About 40 defendants allegedly affiliated with far-right groups like the Proud Boys, the Oath Keepers and the Three Percenters have been charged with conspiring either to impede Congress or law enforcement officials protecting the Capitol.