Bonanza V35B pilot reported failure of one magneto during cruise. Pilot diverted to a nearby airport and a post flight maintenance inspection revealed the Surefly magneto P-lead was grounding resulting in the magneto's failure to fire.

Narrative

We departed ZZZ for ZZZ1 in a Bonanza V35B. We had filed a route which kept us over the Interstate for additional landing options. My non-pilot wife was in the right seat.We were in level cruise flight at 6;000 ft (our final altitude due to potential icing above); in clouds / IMC at +4C. About 30 minutes after departure; our Horizon digital tach indicated a momentary reduction in RPM on the right magneto and then the right red light illuminated; indicating a loss of sensing on the right magneto. Our aircraft is equipped with a Surefly electronic ignition on the right side; and a standard Bendix magneto on the left side. The indication suggested a failure on the Surefly.We did not experience any engine roughness or notable change in power. We were in gliding distance of ZZZ2 airport; but it was reporting 600ft ceilings. 8 minutes ahead of us was ZZZ3; which was reporting 2200ft ceilings (via ForeFlight and ADS-B in).At the time; we were operating lean-of-peak at 16 GPH and ~1600F TIT. About 20 seconds after the initial indication; as I was considering options; I noticed TIT rising; and so I went full rich. This slightly roughened the engine; but not to any significant concern. TIT / EGT returned to acceptable levels. The JPI 700 engine instrument did not show any problems on individual cylinders nor did we have any abnormal oil indications.I had not experienced this issue before.We notified ZZZ Approach of a possible magneto problem and requested direct ZZZ3; which we were immediately given. A few minutes later; about 4 miles from ZZZ3 we broke out into 10+SM VMC. There were a few scud clouds at ~500 AGL; in the vicinity; but not directly over the field. We were cleared to descend to 4000ft; and lateral clearance for air work anywhere within the class Delta.Once directly over the field; we did a magneto check; and as expected; when selecting the right magneto only; the engine began to stop and would have quit without returning to ignition to both. We reported the failed magneto and requested to land at ZZZ3.At the time; we were in a left turn to keep in gliding distance of the field. ZZZ Approach advised that the field was now reporting IFR and that we would need to plan the approach. I recall asking if the tower was opened to confirm IFR status. I don't recall the answer; but recall being told to plan the approach. As we continued the left turn; I had the entire field and both runways in sight; and knew that there was rain and IMC a few miles to the east. I had not had time to brief an approach; nor look up tower frequencies. While the engine was not rough; it wasn't at typically smooth levels; and I could see that while the runways were clear of clouds; there were clouds moving in and out of the area.I was only giving about 10% of my attention to the radio; and so the details are fuzzy. I recall being in the turn on a high left base; and ATC talking about an instrument approach. I told ZZZ Approach we were [requesting priority handling] and that we planned to land on Runway XX ahead of us. We were cleared to land; although the details are fuzzy as we began to quickly prepare for landing. The landing was uneventful; we stopped before the intersecting runway; and we were advised to contact ground. It took me a minute to compose myself on the runway and to lookup the ground frequency (I had switched to the wrong one). Ground gave us taxi instructions to the ramp; and we provided our souls / fuel as requested to close the loop on the communication.I phoned the tower from the FBO to ensure that they didn't need anything from me. They told me that I could have opted to cancel IFR; then request a special VFR clearance; but that they didn't need any other information. (They may have thought I was exaggerating the issue and just didn't want to fly an approach).A local mechanic reviewed the electronic magneto and identified the issue causing the P-lead on the Surefly to be grounded and not to fire. We did an extensive run-up and confirmed that both ignition sources were now functioning; and returned home to ZZZ1.I don't believe that I violated any FARs during the sequence; however it's possible that I descended below my clearance limit while remaining in gliding distance of the field and deciding to land. Lessons learned? After going full rich on the mixture; I probably should have leaned it a bit. I think it would have run a bit more smoothly; given prior experience with this turbonormalized engine. If there had been traffic at the field; my [request for priority] might have caused more disruption. I believe it was the right call; but I probably could have given more heads-up that with a magneto issue; I might become an emergency. Even the minute I waited before diverting to ZZZ3 probably should have been shorter.