The massacre stalled a race for the White House that has become increasingly heated in recent weeks. President Barack Obama, Republican rival Mitt Romney and their advisers are now weighing how soon after the shootings to resume their attacks or whether to temper the tenor of the campaign.
"There's not a playbook for this," said Jen Psaki, Obama's campaign spokeswoman. "Just like everybody, we're taking this day by day."
Both campaigns were keeping their largely negative television advertisements off the air in Colorado, a key battleground state in the November election. The Obama campaign said it would not advertise in Colorado for the rest of the week; Romney campaign spokeswoman Andrea Saul said the Republican's ads in the state would be down at least through Monday.
The campaigns pulled the ads Friday, part of a widespread dialing back of election year politicking after shootings that left 12 dead and dozens more injured. The candidates and their surrogates also canceled campaign events and media interviews for much of the weekend.
Romney made a low-key return to political activity Sunday night in northern California, where he courted Republican donors at three campaign fundraisers. Romney told supporters he would tone down his political rhetoric, at least for the night, in "keeping with the seriousness of the day." The former Massachusetts governor avoided attacking Obama by name.
Obama on Sunday dashed for Aurora, Colo., the site of the shooting. He spent nearly three hours in emotional private meetings with the families of the dead and also met with some of those injured in the attacks.
"I hope that over the next several days, next several months, we all reflect on how we can do something about some of the senseless violence that ends up marring this country," Obama said.
Romney said Obama's trip was "the right thing for the president to be doing on this day."
From Colorado, Obama flew to San Francisco to start a previously scheduled three-day trip that includes a speech to the Veterans of Foreign Wars in Reno, Nev., campaign fundraisers in California, Oregon and Washington state, and a speech to the National Urban League convention in New Orleans.
But the campaign cancelled a rally planned for Portland, Ore. Officials said they felt the tone of the larger event would be inappropriate.
Romney also spent Sunday night in San Francisco following his fundraisers. Like Obama, he was to return to campaigning in earnest Monday, with a roundtable discussion with small business leaders and a fundraiser in southern California.
Much of Romney's week will be focused on America's role abroad. He'll deliver a foreign policy-focused speech at the VFW convention Tuesday. The next day he will launch his first trip abroad as the presumptive Republican presidential nominee. He's expected to attend the opening ceremonies of the Olympics in London and meet with foreign dignitaries in England, Israel and Poland.
The Colorado shooting halted the campaign in the midst of a particularly negative stretch. With the race deadlocked less than four months from Election Day, both camps have been ramping up their attacks as they seek to gain an edge.
The Obama campaign, trying to overcome the weak U.S. economy, has pummeled Romney's business record and financial secrecy. Led by Obama himself, the campaign has pushed the wealthy Romney to release more of his personal tax returns and raised questions about whether his tenure at the helm of private equity firm Bain Capital coincided with outsourcing and bankruptcies.
Romney responded with his own harsh criticism, attacking Obama as anti-private sector for saying: "If you've got a business, you didn't build that. Somebody else made that happen." The Romney campaign used the phrase in an ad last week.
Friday's shooting also put fresh focus on the issue of gun control, which has played virtually no role in a race focused on the economy.
Last year, following the killing of six people and wounding of then-Rep. Gabrielle Giffords in Tucson, Ariz., Obama called for a series of steps to "keep those irresponsible, law-breaking few from getting their hands on a gun in the first place." But Psaki, his campaign spokeswoman, said it was "too early to say" whether the president would propose any new gun control policies following the Colorado shootings.