Sens. Marco Rubio, R-Fla., and James Inhofe, R-Okla., joined three other Republican senators threatening to filibuster any new restrictions on guns Thursday.
The two senators added their signatures to a letter previously signed by Sens. Rand Paul, R-Ky., Mike Lee, R-Utah, and Ted Cruz, R-Texas, vowing to protect the Second Amendment.
The letter, which was originally sent to Senate Majority Leader Harry Reid on March 22, outlines the senators' intent to "oppose any legislation that infringe on the American people's constitutional right to bear arms."
In a statement released Thursday, Rubio said he will oppose any legislation that could be used as a vehicle to impose new restrictions on "responsible, law-abiding gun owners."
"We should work to reduce tragic acts of violence by addressing violence at its source, including untreated mental illness, the lack of adequate information-sharing on mental health issues, and the breakdown of the family,” Rubio said.
Rubio and Inhofe joined the filibuster effort on the same day that President Obama urged Congress not to forget the heartbreak of the Newtown elementary school massacre and "get squishy" on tightened gun laws.
"Shame on us if we've forgotten," Obama said at the White House, standing amid 21 mothers who have lost children to shootings. "I haven't forgotten those kids."
More than three months after 20 first-graders and six staffers were killed in Newtown, Conn., Obama urged the nation to pressure lawmakers to back what he called the best chance in over a decade to tame firearms violence.
Following Thursday's White House event, Cruz accused the president of using the Newtown school shooting for political gain.
"It is saddening to see the president today, once again, try to take advantage of this tragic murder to promote an agenda that will do nothing to stop violent crime, but will undermine the constitutional rights of all law-abiding Americans," Cruz said in a statement.
At the same time, gun control groups were staging a "Day to Demand Action" with more than 100 rallies and other events planned from Connecticut to California. This was on top of a $12 million TV ad campaign financed by New York City Mayor Michael Bloomberg that has been pressuring senators in 13 states to tighten background-check rules.
The Associated Press contributed to this report.