A rise in the number of voters registering as Democrats and a corresponding fall in those preferring Republican Party during the last three years is upsetting electoral balance between the two parties and could affect upcoming electoral cycles.

While the implication of the changing electoral landscape is far from clear for Barack Obama and John McCain -- current contender for the White House -- voting experts said registration numbers may signal the beginning of a move away from Republicans.

Already, there has been a sharp reversal for Republicans in many legislatures and governors' mansions, in several states, including traditional battlegrounds of Nevada and Iowa, Democrats have surprised their own party officials with significant gains in registration.

Swings in party registration are not uncommon from one year to the next, or even over two years.

Registration, moreover, often has no impact on how people actually vote, and people sometimes switch registration to vote in a primary, then flip again come Election Day.

But for a shift away from one party to sustain itself the current registration trend is now in its fourth year is remarkable.