NEW HAVEN, Conn. (AP) -- Six months after the Newtown school shooting, New York Mayor Michael Bloomberg is stepping up his gun control campaign by asking donors not to support Democratic senators who opposed a bill to expand background checks on gun buyers, while a mayors' group he co-founded is embarking on a national bus tour to rally for efforts to curb gun violence.
Some of the victims' families also vowed to keep up the pressure for stricter gun control as they met with lawmakers in Washington.
"This mother's heartbreak that I carry, this life sentence that I have, no one should ever have to bear this burden," said Nelba Marquez-Greene, whose daughter, Ana, was among 26 people killed inside Sandy Hook Elementary School on Dec. 14.
Friday will mark six months since the massacre, and the politics of gun control is mixing with emotions stirred by the tragedy as family members and advocates prepare for remembrances around the country.
A moment of silence is planned Friday in Newtown, where names of victims of gun violence are to be read, followed by a rally in which proponents plan to call for Congress to pass legislation expanding background checks.
In a letter, Bloomberg asked New York City donors not to support Democratic senators who opposed a bill to expand background checks, which helped lead to the legislation's defeat in April. The letter names Sens. Mark Pryor of Arkansas, Heidi Heitkamp of North Dakota, Max Baucus of Montana and Mark Begich of Alaska.
"I am writing to ask you: The next time these four senators want you to support them with donations to their campaigns, tell them you cannot," Bloomberg wrote to about 1,100 New York City residents who had contributed to the four senators. "Until they show they will stand up for the American people and not the gun lobby, tell them you cannot support their candidacy."
Pryor has said he didn't vote for the background check measure because he believed a separate gun control measure he supported by Republican Sen. Charles Grassley of Iowa was stronger. He also said the bill would not have prevented Newtown and other mass shootings.
Heitkamp said she felt the bill would place undue burdens on law-abiding citizens. Begich said the measure would have undermined Second Amendment rights. Baucus said he was voting in accordance with what his constituents wanted.
Marquez-Green was among eight relatives of Newtown victims who met Wednesday with Sen. Joe Manchin, D-W.Va., who co-sponsored the bill to expand background checks that was defeated in April.
Asked about Bloomberg's letter, Manchin said he would prefer to see something "in more of an informational, educational and supportive role" to help build support in states with strong gun cultures.
"I know they're doing what they think is the right thing to do," Manchin told reporters in his office. "We just need more support in educating the public about the virtues of this bill."
The National Rifle Association is targeting Manchin with a TV ad launched because of his support for the expanded background checks. It urges viewers to phone Manchin's office and tell him "to honor his commitment to the 2nd Amendment."
The NRA plans to spend $100,000 airing the ad in West Virginia markets over the next two weeks.
Newtown families also met for nearly an hour Wednesday with House Speaker John Boehner, R-Ohio, and others in the House, where leaders have expressed interest in strengthening mental health programs but not in expanding background checks.
"I thanked them for their courage and willingness to come forward and talk to us," House Majority Leader Eric Cantor, R-Va., said. "I know that they're continuing to seek how to solve the problem and we're going to continue to go and to listen to them and work" on the issue, he said.
The gunman in Newtown killed his mother and then 20 children and six adults at Sandy Hook before committing suicide as police arrived.
The Mayors Against Illegal Guns bus tour, which leaves Friday from Newtown, will last 100 days and stop in 25 states. Gun violence survivors and relatives of victims will rally for support of background checks and other measures they say will reduce gun violence.
Erica Lafferty, whose mother, Dawn Hochsprung, was the principal of the school and was among those killed, vowed to keep up the pressure as well. Lafferty's mother died after lunging at the gunman to try to stop him from firing.
Lafferty said 33 people get killed with guns daily in the U.S. "That's more than one Newtown a day," she said during a conference call with reporters.
"I'm not going to give up until something happens to reduce that number of 33 people a day that are murdered with a gun, 33 families a day should not have to live with the feeling that I feel every single second of every single day," Lafferty said.
Associated Press writers Alan Fram in Washington, D.C., Jennifer Peltz in New York, and Lawrence Messina in Charleston, W.Va., contributed to this story.