Since rallies are paid for by the campaign, though, they do have a right to give tickets out to whomever they please.
You do have to obtain the tickets in advance via the local Republican HQ. And they can check your voter's registration since that's info any committeeman can obtain through the county.
But I've attended a number of rallies for Bush and Cheney here in FL in 2000 and I've often found myself seated next to or standing in line next to Democrats and independents.
If you have an active Republican friend you can gain admission.
Of course, for the most part these rallies are repetitious and you can hear the same speeches on C-Span over and over at different campaign rallies with rallies differing only because of different local politicians and other speakers introducing the main candidate plus if you watch them on C-Span you don't have to stand in line.
Of course, if you plan on toting in a sign declaring your love for Saddam Hussein and a megaphone to shout the famous universal liberal slogan "f**** you" people might not appreciate it and might hogtie you and drag you back to your car along the roughest portion of a gravel road for rudely interrupting their [not your] party. Some liberals find this more exciting than sex and just can't help themselves. That's OK, some of us conservatives like steer roping events. ;-)
My first question to her, was, why do you want to go? you know Bush is preaching to the choir. Do you want to be surrounded by Republicans? i figured that would make her skin crawl.