HARRISBURG — Pennsylvania’s five most recent former governors have collectively announced their support for open primaries, adding new fuel to a long-smoldering debate over whether the commonwealth should allow unaffiliated voters to help choose partisan candidates.

In a letter published Monday, Govs. Tom Corbett, Ed Rendell, Tom Ridge, Mark Schweiker, and Tom Wolf wrote that they, “pledged to govern on behalf of all Pennsylvanians,” and argued that primary elections are now decided by fewer, more extreme voters. The letter goes on to assert that opening primaries would help tamp down extremism and polarization.

Pennsylvania is one of nine states with a closed primary system. Voters in the state can cast primary election ballots only for the party to which they are registered, which crucially leaves the commonwealth’s roughly one million independent or unaffiliated voters unable to participate in major party primaries at all. An additional 300,000 voters registered with third parties are also affected.

Independent and third-party voters are still able to participate in statewide referenda, other local ballot initiatives, and special elections that coincide with partisan primary elections.