Yep, Ebsen was a Republican. When his castmate Nancy Kulp (Miss Jane, the bank president's secretary) ran on the Democrat ticket for the Senate seat in Pennsylvania, she had a radio commercial done which made it sound as if all her surviving castmates supported her candidacy (I'm not sure but I think Max Baer, Jr. and Donna Douglas were also Republicans, but just not as vocal as Ebsen was about their politics). So Ebsen had a commercial done for Kulp's Republican oppinent. This infuriated Kulp. And she lost. :-)
She was running for Congress from the Pennsylvania 9th district against Bud Shuster, not for a US Senate seat. In 1984 Shuster was running for his 7th term of his 14-term congressional career. From Wikipedia:
Shuster was frequently unopposed for re-election. His most notable challenger came in 1984 when Nancy Kulp, the actress who played Miss Jane Hathaway on The Beverly Hillbillies won the Democratic nomination. Kulp, a native of Pennsylvania, had returned to her home state upon her retirement from acting and received support from her friends in Hollywood. This prompted Shuster to recruit Kulp's Hillbillies co-star Buddy Ebsen, a Republican, to record radio spots declaring, "Hey Nancy, I love you dearly but you're too liberal for me — I've got to go with Bud Shuster." Shuster went on to win re-election with two-thirds of the vote.I doubt she would have won even without Buddy Ebsen's help to Shuster. Ronald Reagan may have not had coattails when he ran for reelection in 1984, but congressional Republican incumbents didn't have trouble getting reelected then either.