Why they did it

McClatchy:

Last April, while the Justice Department and the White House were planning the firings, Rove gave a speech in Washington to the Republican National Lawyers Association. He ticked off 11 states that he said could be pivotal in 2008. Bush has appointed new U.S. attorneys in nine of them since 2005: Florida, Colorado, Wisconsin, Minnesota, Iowa, Arkansas, Michigan, Nevada and New Mexico. U.S. attorneys in the latter four were among those fired.
Rove thanked the audience for "all that you are doing in those hot spots around the country to ensure that the integrity of the ballot is protected." He added, "A lot in American politics is up for grabs."
... Despite the GOP concerns, Bud Cummins, the Republican-appointed U.S. attorney in Arkansas who was fired, said he had "serious doubts" that any U.S. attorney was failing to aggressively pursue voter fraud.
"What they're responding to is party chairmen and activists who from the beginning of time go around paranoid that the other party is stealing the election," Cummins said. "It sounds like to me that they were merely responding to a lot of general carping from the party, who had higher expectations once the Republican appointees filled these posts that there would be a lot of voting fraud investigations.
"Their expectations were unrealistic."

It's only paranoia in the case of the GOP. They know they can only win by cheating