Is Scott Walker about to be embroiled in yet another corruption probe?

The Milwaukee Journal Sentinel reported Monday that a secret investigation has been launched into possible campaign violations. The Wisconsin governor, who has survived a recall election and a previous probe relating to his conduct while serving as Milwaukee County executive, may be under investigation in connection to activity that has occurred since he was elected governor in 2010, particularly during the 2012 recall election.

Although the probe’s existence was just reported this week, it was apparently launched more than a year ago and stayed a secret until now under a Wisconsin law allowing for “John Doe investigations.” These probes are similar to grand jury proceedings but operate in secret, and prosecutors have the ability to grant witnesses immunity in exchange for testimony.

Walker repeatedly refused to answer questions about the investigation at a public event earlier this week, saying, “In terms of a lot of the questions regarding that issue, I really don’t have a lot to say.” He went on to describe the probe as a “sidebar issue” and hinted it was politically motivated. Walker’s office did not respond to a request for comment from The Daily Beast.

The Journal Sentinel also reported that the conduct of “a current legislative leader,” widely believed to be Republican Senate Majority Leader Scott Fitzgerald, is part of the investigation as well. Fitzgerald did not return a request for comment.

Walker became a national political celebrity after successfully pushing anti-union legislation through the Wisconsin statehouse and then surviving the subsequent recall election. His name has since been mooted as potential Republican presidential candidate in 2016, and he has visited Iowa several times this year. But he has long been under scrutiny from state investigators. Ethical clouds from his tenure as Milwaukee County executive have long proved politically problematic, with a long-running criminal probe of his appointees leading to several convictions for crimes, including embezzlement and campaign ethics violations. Walker long maintained that he was never a target in the criminal investigation, which was eventually shut down in March 2013.

But the newly public investigation could take some of the shine off Walker’s popularity with conservatives. Political pundits are likely to see connections to two wide-ranging ethics investigations in the same year as a poor launching pad for a presidential campaign. Before any national campaign, however, Walker must survive a 2014 reelection campaign against Mary Burke, a well-funded former state commerce secretary whose family founded Trek Bicycle.

Considering the probe’s existence just became public this week, more than 18 months after it was launched, more details about the investigation are not expected to leak. But whenever prosecutors unseal whatever evidence they’ve accumulated, the impact is likely to spread beyond Wisconsin.