Of all the major terrorist attacks or offensives in history, here's the ones nobody talks about, and were all Leftwing:

• Wall Street Bombing - 30 killed and over 100 wounded in the largest terrorist attack of it's time
• Sterling Hall Bombing - 1 killed, but the Communist conspirators almost entirely got off. The primary conspirator served only 7 years, became a hot dog vendor, and said he'd do it again. It was the inspiration for the OKC bombing.
• Burning of Kent State ROTC building - Attempt to murder 300 cadets while they slept, and is why the National Guard were at Kent State.
• 1954 Congressional Shooting - Puerto Rican Separatists attempted to machine gun the entire US Congress wounding 5 people
• MOVE Firefight (1978) - Black Separatist Communist organization takes over a portion of Philadelphia in an insurrection and kills 1 police officers and wounds 13 others in a major gunbattle
• MOVE Siege (1985) - The same group does the same thing, but this time after another major gunbattle, they are hit with an incendiary bomb to force them out of the building. You always hear about the bombing, but never why.
• Attica Prison Hostage Crisis (1971) - 43 Killed, 42 hostages, 90 Wounded. Bloodiest prison "riot" that I can even think of in US history

And just to be clear only The Wallstreet Bombing is even referred to as a Leftist Terrorist attack on Wikipedia.