Rick Ungar at Forbes reports:
Of the eight GOP state senators who would be subject to a recall action, three of them won their election with 51% of the vote or less. Were those three senators to be recalled and replaced with democrats, that would be all that is required to give control of the senate to the Democrats.
Wikipedia confirms Ungar's numbers. The three vulnerable Republican state senators are Alberta Darling (SD-8), Randy Hopper (SD-18), and Dan Kapanke (SD-32). Darling and Kapanke each garnered 51% of the vote while Hopper squeaked by with 50%.
Darling lives in and represents River Hills, a northern, wealthy suburb of Milwaukee. She narrowly defeated Sheldon Wasserman, a former state assemblyman in a two-way race. Hopper represents Fond du Lac, a small industrial city north of Milwaukee, and narrowly defeated Democrat Jessica King in a three-way race (a Libertarian candidate accounted for less than one percent of the vote). Kapanke represents LaCrosse, one of the major cities in Western Wisconsin, won re-election by a narrow margin in 2008, and then was defeated in a challenge to Congressman Ron Kind (WI-03) in 2010.
Kapanke has represented SD-32 since 2004, Darling has been in office since 1992, and while Hopper is in first term his district has been represented by a Republican since at least 1976. None of these will be easy wins, but they are all within reach given the upheaval Walker's intransigence has caused.