Politicians on Twitter and in written statements reacted with horror to the school shooting that left 27 people dead, including 18 children, in Newton, Conn., Friday.

Mark Kelly, husband of former Congresswoman Gabrielle Giffords, who survived a shooting at a Tucson shopping mall in January 2011, said it is time for U.S. officials "to stand up and do what is right."

"This time our response must consist of more than regret, sorrow, and condolence. The children of Sandy Hook Elementary School and all victims of gun violence deserve leaders who have the courage to participate in a meaningful discussion about our gun laws - and how they can be reformed and better enforced to prevent gun violence and death in America," Kelly wrote on his Facebook page.

New York Mayor Bloomberg, one of the co-chairs of Mayors Against Illegal Guns said he is determined "to stop this madness."

"With all the carnage from gun violence in our country, it's still almost impossible to believe that a mass shooting in a kindergarten class could happen. It has come to that," Bloomberg said in a statement. "President Obama rightly sent his heartfelt condolences to the families in Newtown. But the country needs him to send a bill to Congress to fix this problem. Calling for 'meaningful action' is not enough. We need immediate action."

New York Gov. Andrew Cuomo said he was "shocked and saddened" by the tragic shooting. He said society should "unify" to "crack down on the guns."

"During times of such unthinkable tragedy, all New Yorkers stand together with the people of our neighboring state to grieve the loss of life and help bear the pain and anguish that will be felt by so many in the weeks, months and years to come," Governor Cuomo wrote in a statement. "Let this terrible tragedy finally be the wake-up call for aggressive action and I pledge my full support in that effort."

Rep. Jerrold Nadler, D-N.Y., also called immediately for tougher gun laws. He said he was "absolutely horrified" by news of the shooting at Sandy Hook Elementary School.

"Yet another unstable person has gotten access to firearms and committed an unspeakable crime against innocent children. We cannot simply accept this as a routine product of modern American life," Congressman Nadler said in a statement.

Nadler reacted to White House Press Secretary Jay Carney 's assertion that today is not the day to discuss gun control policy. Carney was asked if today's shooting makes "limiting handgun violence or other gun violence" a higher priority for the president.

"There is, I'm sure - will be, rather, a day for discussion of the usual Washington policy debates, but I don't think today is that day," Carney said.

"If now is not the time to have a serious discussion about gun control and the epidemic of gun violence plaguing our society, I don't know when is. How many more Columbines and Newtowns must we live through? I am challenging President Obama, the Congress, and the American public to act on our outrage and, finally, do something about this."

New York Congresswoman Nita Lowey echoed Nadler's sentiments in a statement: "Our expressions of sympathy must be matched with concrete actions to stop gun violence."

Virginia Gov. Bob McDonnell related today's events to the shooting that took the lives of 32 people at a university in his state.

"My thoughts and prayers go out to the families of those impacted by the events transpiring today, and to the teachers, emergency responders, and all others touched by this tragedy. Unfortunately, Virginia has our own painful memories of the tragic shootings at Virginia Tech in 2007. Those memories will never fade, and we continue to grieve for all those lost on that April day," Governor McDonnell wrote in a statement. "We are all too aware of the impact that events like this can have on a community. If there is anything Virginia can do to assist Governor Malloy and the citizens of Connecticut, we stand ready to do so."

Gov. John Hickenlooper of Colorado - where the Columbine High School shooting took 13 lives in 1999, and where this past July, 12 were killed by a gunman in an Aurora movie theater - also offered his condolences.

"The shooting in Connecticut is absolutely horrific and heartbreaking," the Colorado governor wrote in a statement. "We know too well what impact this kind of violence has on a community and our nation. Our thoughts and prayers are immediately with the families of those killed. We can offer comfort, but we all know the pain will stay forever."

More than 70 members of the House of Representatives used Twitter to express their sadness over the violence in Connecticut.

Rep. Joe Courtney, R-Ct., tweeted his reaction.

#Newtown shooting is an horrific, senseless tragedy. Thoughts and prayers for victims, families, and the Sandy Hook Elementary community.

— Rep. Joe Courtney (@RepJoeCourtney) December 14, 2012

House Speaker Nancy Pelosi predicted the nation will support Newton in the weeks to come in her statement.

No words can console the parents of the children murdered at Sandy Hook. We share our prayers and our grief over these horrifying events.

- Nancy Pelosi (@NancyPelosi) December 14, 2012

ABC's John Parkinson, Elizabeth Hartfield and Sunlen Miller contributed to this report.