INDIANAPOLIS  Until now, Shirley Morgan had always been the kind of voter the Republican Party thought it could count on. She comes from a family of staunch Republicans, has a son in the military and has supported Republican presidential candidates ever since she cast her first ballot, for Richard M. Nixon in 1972.

But this year Mrs. Morgan exemplifies a different breed: the Republican crossing over to vote in the Democratic primary. Not only will she mark her ballot for Senator Barack Obama in the May 6 primary here, but she has also been canvassing for him in the heavily Republican suburbs of Hamilton County, just north of Indianapolis  the first time she has ever actively campaigned for a candidate.

“I used to like John McCain, but he’s aligning himself too closely with what Bush did, and that’s just not what I want for this country,” Mrs. Morgan, who is 56, said when asked to explain her rejection of the presumptive Republican nominee.

Since the start of the primary and caucus season in January, Republican voters have been crossing over in increasing numbers to vote in Democratic contests  supplying up to 10 percent of the vote in states that allow such crossover voting  and they are expected to play a pivotal role in the fiercely contested primary here. What is less clear, however, is the motivation for their behavior: are they genuinely attracted by the two Democratic candidates? Or are they mischief-making spoilers, looking to prolong a divisive Democratic fight or support a candidate Mr. McCain can beat in November?