The Florida and Oklahoma Republicans, respectively, joined Sens. Rand Paul of Kentucky, Ted Cruz of Texas and Mike Lee of Utah in vowing to oppose "any legislation that would infringe on the American people's constitutional right to bear arms, or on their ability to exercise this right without being subjected to government surveillance," said a letter Rubio and Inhofe signed.
The letter, dated a week ago, was sent to Senate Majority Leader Harry Reid, D-Nev., and was originally signed only by Paul, Cruz and Lee. It still has seven blank lines available for other signatures, a United Press International review of the letter indicated.
A filibuster is a way for senators to delay or prevent a vote on a bill by speaking for as long as they wish on any topic they choose, unless 60 senators bring debate to a close.
Paul -- a closely watched conservative who, like Rubio, is considering a 2016 presidential run -- led a nearly 13-hour filibuster on the CIA's drone policy early this month, forcing the Senate to delay the confirmation of John Brennan to lead the CIA.
The Senate is expected to debate a gun-violence bill after it returns from Easter recess April 8.
The bill, put together by Reid from Senate Judiciary Committee proposals, is expected to include a Democratic plan to expand the nation's gun background check program, reauthorize a Justice Department school-safety grant program and toughen federal laws on gun trafficking.
Reid has said a ban on certain styles of semiautomatic so-called assault weapons is virtually assured of defeat.
"We should look for ways to keep firearms out of the hands of criminals and the mentally ill prone to misusing them, but I oppose legislation that will be used as a vehicle to impose new Second Amendment restrictions on responsible, law-abiding gun owners," Rubio said in a statement.
Rubio and Inhofe announced their intention to join a possible filibuster Thursday shortly after President Barack Obama urged congressional lawmakers to pass stricter gun laws a day shy of 15 weeks after the Dec. 14 mass shooting at Sandy Hook Elementary School in Newtown, Conn., that killed 20 schoolchildren and six adult staff members at Sandy Hook Elementary School.
"Less than 100 days ago that happened, and the entire country was shocked," Obama said at the White House, surrounded by relatives and friends of gun-violence victims, including some from Newtown. "And the entire country pledged we would do something about it and that this time would be different.
"Shame on us if we've forgotten. I haven't forgotten those kids. Shame on us if we've forgotten," Obama said.
Cruz followed Obama's impassioned remarks with a statement.
"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, promising to "use any procedural means necessary to protect those fundamental rights."
At the same time, Sen. Chuck Grassley, R-Iowa, began floating a competing gun bill that opposes expanded background checks for gun sales -- a key provision of the Democratic gun bill.