When President Obama made his state of the union address in February, he had a simple message to deliver to Congress. In the wake of the Newtown shooting, in which 20 children aged five and six were killed, it was time for Congress to debate commonsense proposals to reduce the threat of gun violence and at the very least to vote for them.

"The families of Newtown deserve a vote," he said.

Less than two months after Obama delivered that clarion call, 13 Republican politicians in the US Senate are preparing to do precisely the reverse. They have indicated that when a comprehensive reform bill aimed at preventing future gun rampages like Newtown is presented to the Senate next week, they will impose a filibuster that will be virtually impossible to overcome.

In other words, there will be no vote.

The decision of the Republican leader in the Senate, Mitch McConnell, to support the 13 and block a vote on gun reforms shocked even seasoned observers of Congress's resistance to change when it comes to guns, including some senior figures within the Republican party itself. As John McCain, a former Republican presidential nominee, put it: "I don't understand it. The purpose of the US Senate is to debate and to vote and to let the people know where we stand."

A filibuster, which would be unbreakable unless the Democratic leadership can muster 60 votes, would allow wavering politicians to avoid having to reveal where they stand in the fraught gun debate. The National Rifle Association, the main pro-gun lobby group, is famously aggressive at policing Congress members, awarding each one a ranking based on their voting record and whether they are robust in defending the rights of gun owners.

Now Mayors Against Illegal Guns, the coalition headed by New York's Michael Bloomberg, has announced that it is to launch its own ranking system but from the opposite stance. It will be scoring each politician according to whether or not they back what the group calls "sensible" gun control measures.

But the NRA's ranking system, and Bloomberg's opposing scorecard, are irrelevant if Congress members do not vote at all. In his second speech in Connecticut since the 14 December Newtown tragedy, Obama lashed out on Monday night against the threatened filibuster.

"Think about that. They're not just saying they'll vote no on ideas that almost all Americans support. They're saying they'll do everything they can to even prevent any votes on these provisions. They're saying your opinion doesn't matter, and that's not right," he said.

Meet the 13 Republican senators who in a 8 April letter to Senate majority leader Harry Reid of Nevada pledged to block a vote on what they called "any legislation that would infringe on the American people's constitutional right to bear arms". They include two leading Republican hopefuls for the presidential race in 2016, Rand Paul and Marco Rubio; three of them enjoy A+ rankings from the NRA and a further eight are listed A.

Rand Paul of Kentucky

The junior senator's penchant for filibusters became famous during his nearly 13-hour speech against the use unmanned drones, and he is one of three senators who sent an initial missive to Reid, warning him of another verbose round. Paul's independent streak, libertarian leanings and his recent CPAC performance have made him popular among Tea Partiers and young conservatives, but haven't garnered him much support in the party. Senator John McCain called Paul and cosignatory Ted Cruz "wacko birds" for their refusal to debate certain issues.

Ted Cruz of Texas

Senator Ted Cruz speaks during a news conference at the Capitol in Austin, Texas, on Monday, April 1, 2013. Photograph: Deborah Cannon/AP

A Tea Party favorite who has had to contend with his own "birthers", the Texas senator has defended his hard-right ideology with an occasionally abrasive fervor. His argumentative style soured a recent debate with senator Dianne Feinstein, in which he spent several minutes attacking her ban on assault weapons and describing the constitution. Feinstein responded: "I'm not a sixth-grader … I am reasonably well educated, and I thank you for the lecture."

Mike Lee of Utah

The third signatory of the original letter, the junior senator from Utah has made gun rights one of the primary issues of his young congressional career. A former constitutional lawyer, he opposes any changes to regulation in the US, arguing that new rules will unfairly affect law-abiding citizens. He also spoke out against the UN treaty to regulate the global arms trade – a treaty that North Korea and Syria voted against.

Marco Rubio of Florida

Marco Rubio. Photograph: Jacquelyn Martin/AP

Chosen to make the minority rebuttal to President Obama's state of the union address, Rubio has been seen as a frontrunner to lead a younger, more diverse GOP. He has signed onto the second letter in opposition to any gun control bill, but despite his conservative voting rhetoric, he has not taken to the vitriolic language of Ted Cruz or the defiant attitude of Rand Paul. Party leaders like John McCain and Lindsey Graham have thrown him their support, and his stance on immigration has made him look moderate compared to his peers. Accordingly, he scored a B+ from the NRA.

James Inhofe of Oklahoma

Osama bin Laden is dead. 'He's history,' says Senator James Inhofe, above, after viewing photographs of the al-Qaida leader's corpse. Photograph: Susan Walsh/AP

Inhofe has been a conservative staple of the Capitol for two decades, expounding his far-right views on social issues and foreign policy since 1986. He famously continues to deny climate change and proposed an immigration amendment to make English the national language of the US. He is only one of two senators to have received an A+ from both the Gun Owners of America and the NRA, and he has voted to exempt gun manufacturers from lawsuits concerning injury or death due to product defects.

Richard Burr of North Carolina

Since he took office in the House in 1995, Burr stayed a party man for the GOP when he attained his Senate seat a decade later. Burr has supported deregulating everything from tobacco to oil, and he received an A from both the NRA and the Gun Owners of America. His most notable deviation from the party line was with several other senators who voted to repeal "don't ask, don't tell" in 2010.

Mike Enzi of Wyoming

A senior senator from Wyoming, Enzi worked for the Department of Interior and the private Black Hills Corporation before being elected to Congress. His stance on guns follows suit with his conservative social positions; Enzi has voted to ban gay marriage and burning the flag, for instance, and his votes on gun rights earned him an A grade from the NRA.

Jerry Moran of Kansas

Elected to the Senate in the Tea Party rush of 2010, Moran has largely spent his congressional career on matters of deregulation and agriculture. Socially, he has kept to close to the GOP's most conservative contingent, receiving an A from the NRA. On a few policy issues, however, he has strayed from the party line, as when he voted against No Child Left Behind and Sopa.

Pat Roberts of Kansas

The senior senator from Kansas has largely made a name for himself in national security and foreign policy, after he led the investigation into intelligence failures preceding the war in Iraq. Seeking re-election in 2014, Roberts has turned to social issues and quietly pushed the conservative line. Having voted to allow passengers keep firearms in checked baggage on Amtrack trains, he received an A from the NRA.

Ron Johnson of Wisconsin

Johnson has skewed to the Tea Party values that elected him to his first congressional office in 2010, opposing just about any bill that involves government regulation. He has sponsored a bill that would prevent the DOJ from tracking the purchases of multiple rifles and shotguns and received an AQ from the NRA.

Dan Coats of Indiana

Coats has served twice in the Senate, retiring in 1999 only to return in 2011; his policies have inched to the right in the decade-long interim. Though he voted for background checks and Dianne Feinstein's original assault weapons ban – votes that dropped his NRA score to a C+ – he has otherwise consistently voted conservatively on social issues.

Mike Crapo of Idaho

Senator Michael Crapo has been arrested and charged with driving under the influence. Photograph: Matt Cilley/AP

Since he took office in the Senate in 1998, Crapo has kept an eye toward energy, earning the scorn of environmentalists for votes in favor of oil drilling and against funding renewable sources. On social issues, he's voted largely with the far right, and he received an A+ from the NRA for his various votes against regulating guns.

James Risch of Idaho

James Risch Photograph: risch.senate.gov

Idaho's junior senator took office in 2009 after serving as both governor and lieutenant governor. The NRA has given him an A for his votes against background checks and in support of the right to carry concealed weapons across state lines.