Country star Glen Campbell spent his 77th birthday on Monday (22Apr13) advocating for Alzheimer's disease research in Washington, D.C., two years after he was diagnosed with the neurological disorder.

The Rhinestone Cowboy hitmaker, who went public with his battle with short-term memory loss in 2011, has spent the last few months wrapping up his goodbye world tour after suffering from worsening symptoms.

On Monday, he visited U.S. senators at Capitol Hill to encourage government leaders to support the fight to find a cure before heading to a special fundraising dinner for the Alzheimer's Association.

The veteran officially celebrated the milestone at home with friends and family, including his songwriter pal Jimmy Webb, over the weekend (20-21Apr13), and his wife, Kim Campbell, insists the singer remains in good spirits - despite his declining health.

She tells the Associated Press, "We're trying to live our lives and stay out there and socialise as much as we can as long as we can. The other night we went to see Merle Haggard and hung out with Toby Keith on his bus... Everybody played different songs and had a great time. So we're still out there trying to do that kind of stuff. But eventually we might not be able to do that."