The NRA is divided into several organizations that have different tax status and functions. 

Your NRA membership dues go to the main organization, is not used for political purposes,
and is not tax deductible. 

NRA ILA http://www.nraila.org/ is the lobbying branch of the NRA and this is funding used for NRA people in DC and the state capitals who interact with various legislative bodies. 

The NRA PVF http://www.nrapvf.org/ is the fund used for making contributions to political candidates and for political advertising. 

The NRA Foundation http://www.nrafoundation.org/ is tax deductible and it does things like safety and marksmanship training and range programs. This is a Section 501(c)(3) organization making donations tax deductible. 

The NRA Civil Rights Defense Fund http://www.nradefensefund.org/ is the branch that helps with things like law suites and other gun rights court cases. This one is also tax deductible as a Section 501(c)(3) org.

NRA/Friends of NRA use grass roots "Friends Committees" to raise funds and State "Grant Funding Committees" to spend the money raised.

Most membership money goes to your magazine subscription and operations of the club, HQ salaries, etc. Donations account for most other expenses and undertakings.

My question... Why do most people buy an annual membership instead of donating directly to the NRA ILA? I say, "NRA, keep your hats, duffle bags, and magazines. I demand my money go directly to the front lines to fund those that defend our 2A rights!