Sen. Ruben Diaz Sr., a conservative Bronx Democrat, just waltzed through the doors of the Westchester Hilton here in Rye Brook where the post-convention day festivities are in full swing.

Diaz Sr., who was accompanied by his wife, told me this is his first GOP convention. He said he is attending at the invitation of state GOP Chairman Ed Cox and Westchester County Executive Rob Astorino, the Republican gubernatorial candidate who will be officially nominated here tomorrow.

The senator, who has never made a secret of his friendships with Republicans, seemed to feel his presence here – the only Democrat I’ve seen so far – is perfectly normal.

He explained that Cox had co-hosted a New York Hispanic Clergy event with him last Christmas, and Astorino – as well as Senate GOP Leader Dean Skelos – attended the event, too.

I asked the senator several times if he’s planning to cross party lines to endorse Astorino. He’s no fan of his fellow Democrat, Gov. Andrew Cuomo, and in fact is a frequent critic of the governor.

Diaz Sr. didn’t answer that question one way or another, simply reiterating that he had been invited to attend, and so decided to do so.