Gun lobbyists raised record amounts of money for political campaign coffers in the months after the Newtown school shooting, as they attempt to tip the balance on a knife-edge vote on gun control this week.

Federal Election Commission disclosures show the National Rifle Association, which has led efforts to block gun control reform, raised $2.7m for its political action committee in January and February this year – a 350% increase on the same period after the last mid-term elections.

It spent some of the money on contributions to a clutch of Republicans in the House of Representatives, including speaker John Boehner, but has also amassed its largest campaign finance warchest for over a decade, according to analysis by the Guardian.

The Senate will begin debating key legislation on Tuesday that would strengthen background checks on gun buyers and close loopholes that currently allow gun fairs and online sellers to avoid laws banning criminals and mentally ill people from purchasing firearms.

But although the Senate last week voted 68-31 in favour of allowing this debate to go ahead after a compromise between Republicans and Democrats, voting on the amendment itself is expected to be extremely tight.

Several of the 16 Republicans who rebelled against their party leadership to allow the debate, including Lindsey Graham of South Carolina, have since said they will not vote in favour of the legislation.

Other, such as Susan Collins of Maine, have complained of being attacked by TV commercials paid for by the National Association of Gun Rights, a lobby group even more fiercely opposed than the NRA.

Groups favouring gun control have also been spending money, too, with New York mayor Michael Bloomberg financing TV commercials on behalf of a group called Mayors Against Illegal Guns. "I am being besieged by all sides," Collins told NBC.

But proponents of gun legislation are dwarfed by the political muscle of the pro-gun lobby, according to Washington experts.

Kathy Kiely of the Sunlight Foundation said: "Generally, their power has been in the form of threats: 'You don't toe the line, [so] we come after you.' They are circling the wagons because they feel like they are under siege."

"Their influence is not just measured in TV ads," Keily said. "The NRA has tremendous grassroots power because they have this big membership."

In January, it was reported that membership of the NRA grew by 250,000 in the month following the Newtown shooting to over 4.25 million members.

The recruitment drive is likely to have played a big part in the surge in contributions to the NRA's Political Victory Fund, a political action committee that funnels money to politicians.

According to Guardian analysis of the FEC data, it raised $2.7m in January and February, compared with $605,000 during the equivalent non-election year period in 2011. The figure at this stage in the election cycle in 2009 was $1.2m. In the latest available filings, the fund also reported a cash pile worth more than $4m – the highest figure for this period in a non-election year since at least 1999. This time in 2009 it boasted just $2.7m in reserves.

Further NRA finance data for March is due to be released on Saturday, and quarterly filings from politicians were due out later on Monday, which may show which of the swing members of the House and Senate have received fresh money from lobbyists since Newtown.

The biggest fear for politicians is not that the relatively small sums of money given to each of them might dry up – Speaker Boehner received $2,500 from the NRA in January and February – but that its warchest might be turned against them at the next election.

Figures for the two years leading up to November's general election show that another NRA-affiliated lobby group, the Institute for Legislative Action, spent $8.5m on television commercials and telephone drives – the vast majority attacking opponents rather than promoting its own candidates. About $7.5m of this spending was also so-called "dark money", which means there is no need to disclose which politicians were targeted by the ads. Campaign rules mean not-for-profit groups like this cannot give money directly to politicians, but only have to detail spending on independent campaigns that explicitly endorse individual candidates.

Analysis of dark money expenditure by the Sunlight Foundation shows the NRA was behind at least five TV ad campaigns against gun control since Newtown, targeting key swing states such as Ohio.

The NRA has also tightened the screws on senators in recent days by taking the unprecedented decision to award negative scores to anyone who voted for last week's so-called "cloture motion" allowing the gun debate to go ahead.

An estimated 21 senators who voted for cloture previously received top marks by the NRA for their stance on gun rights, an endorsement which can be highly influential during elections.

Two Democrats voted against the cloture vote: Mark Begich of Alaska and Mark Pryor of Arkansas. Both are up for re-election next year in predominantly conservative states. Other red-state Democrats are expected to vote against the amendment on Wednesday, requiring its backers to persuade at least six Republicans to defy the NRA and stick with the compromise agreement for it to pass. They still face even bigger hurdles in the Republican dominated House of Representatives, where much of the NRA's lobbying efforts are focused.

The NRA said it opposed the Senate amendment, and any other anti-gun measures, because it would do little to prevent further mass shootings.

"As we have noted previously, expanding background checks at gun shows or elsewhere will not reduce violent crime or keep our kids safe in their schools," it said in a statement. "Given the importance of these issues, votes on all anti-gun amendments or proposals will be considered in NRA's future candidate evaluations."