The issue of gun control has been one of the most divisive issues in American politics for decades. It emerged again, this time when a gunman killed 17 people at a high school in South Florida — the 18th school shooting this year.

Yet while the debate has generated millions of dollars in campaign contributions and lobbying on both sides, gun rights advocates have sharply outspent groups advocating gun control.

Since 1989, gun rights interests have given about $41.9 million in direct spending to candidates, parties and outside spending groups, according to the Center for Responsive Politics, which tracks data reported to the Federal Election Commission.

Nearly 90 cents of every dollar of that money went to Republicans, according to the CRP. In the 2012 and 2014 election cycles alone, gun interests spent at least another $48 million in so-called outside spending, which is not reported by individual candidates.

Most of the money has come from the National Rifle Association, which has contributed nearly $23 million in direct spending since 1989. During the 2016 election cycle, it put up another $54.3 million in outside expenditures, up from $27 million during the 2014 cycle. Other groups, including the National Association for Gun Rights, Gun Owners of America and the National Shooting Sports Foundation, have also spent millions promoting gun rights, according to the CRP.