Wisconsin Gov. Scott Walker's (R) campaign and a number of conservative groups that backed him against a recall effort are being subpoenaed, according to the editorial page of The Wall Street Journal.

The targets are part of an ongoing secret John Doe investigaton into possible campaign coordination violations. They include Walker's campaign, the Wisconsin Republican Party, the Republican Governors Association,  Americans for Prosperity, American Crossroads and the Wisconsin chapter of the Club for Growth.

This is the second such probe into Walker's campaign finances — the first led to charges against three former Walker aides but didn't find him personally culpable of anything.

The ongoing investigation could complicate Walker's reelection prospects next year, and if he runs for president could play a role. Walker is in the midst of a national swing promoting his new book, and has made it clear he's considering a presidential run.

Walker declined to discuss the ongoing probe when it was first reported, but suggested it was politically motivated.

"From our standpoint, we assume with there now officially being a candidate for governor on the Democrat side, there are going to be stories like this coming out — in some ways, I think, distracting from our good work on improving the economy and providing $100 million worth of tax relief," he said in late October.