A top aide to presumptive Republican nominee Sen. John McCain (Ariz.) said Tuesday that Sen. Barack Obama’s (D-Ill.) controversial remarks about people in small-town America are “an important and defining moment” in the campaign.

McCain and Sen. Hillary Rodham Clinton (D-N.Y.), as well as their surrogates, have heaped criticism on Obama since late Friday afternoon when news broke that the Illinois senator told a crowd at a San Francisco fundraiser that, as a result of prolonged job loss, many small-town Americans have become "bitter," and they "cling to guns or religion or antipathy to people who aren’t like them or anti-immigrant sentiment or anti-trade sentiment as a way to explain their frustrations.”

Campaign aides for McCain and Clinton said Tuesday that they would keep pushing the controversy in hopes of keeping the narrative alive throughout his candidacy.

Obama on Tuesday continued to defend the remarks, telling a union group in Washington that candidates who don't recognize the bitterness are "out of touch."

Clinton's campaign also said Tuesday it would strive to keep the issue alive. Howard Wolfson, a senior Clinton adviser, told reporters "we believe this is an important campaign issue."

He added that the remarks are "something we intend to continue to discuss."

The Clinton campaign has aggressively sought to make Obama's remarks the dominant campaign narrative this week.

Monday night, the campaign went on the air in Pennsylvania, which holds its critical primary April 22, with an ad featuring Keystone Staters who were "offended" by the remarks.

On Tuesday afternoon, the campaign released a list of 100 Pennsylvania mayors who endorsed Clinton. The campaign also released an open letter from 11 mayors from Indiana, which votes May 6, expressing their "sincere disappointment" with Obama's comments.

"These views are offensive to millions of hardworking Hoosiers who strive everyday to provide for our families, lift up our communities and strengthen our country," the letter said.