Glyn Davies, who stood down as Conservative MP for Montgomeryshire ahead of the general election, said that it was one of the most important matters for voters to consider.
He said: "The Nato summit in Watford, involving 29 countries committed to supporting each other if threatened is testament to the key role that the UK plays in defence of the free world.
"Under a Conservative government, the UK has always taken its key role seriously, and always contributed two per cent of GDP to finance it.
"Other countries don’t do that, which causes Donald Trump much angst. But fair play, the US under Trump remains the dominant founder of our safety umbrella, and he is in Watford for the summit.
"Throughout my life’s interest in international affairs, every Labour Prime Minister has been as committed to Nato as every Conservative Prime Minister. I have never imagined that any party leader, in with a shout of reaching No 10 could take a different view. I just feel don’t feel confident that is the case today. In my view this is the most important issue we need to consider before voting on December 12."
The Labour manifesto does stress a commitment to Nato, stating: "We will maintain our commitment to Nato and our close relationship with our European partners."