Cheryl, a lifelong vocal Democrat, was happy and smiling Thursday night during the president-elect's big bash for his cabinet nominees, including RFK Jr. ... his pick for Health and Human Services Secretary.

This was the first time the couple's been seen together at a Trump event since the election, but sources with direct knowledge tell us she's been onboard all along. We're told RFK Jr. went to Mar-a-Lago the day after Trump's landslide victory, and Cheryl was with him as DT offered the HHS Sec. position.

What's interesting is that 9 months ago the Kennedys joined us on "TMZ Live," and Cheryl strongly suggested their marriage might not survive if RFK Jr. became Trump's running mate ... which was being discussed at that time.

Obviously, a lot's changed since then -- RFK Jr. ended his own presidential campaign, made a fiery entrance as a Trump supporter and stumped for him right up through the election ... and Cheryl's apparently softened her view on Trump.

There were also rumors they'd split up when it came out in September he'd allegedly sexted with a New York Magazine reporter, but our sources tell us Cheryl's never waivered on standing by him.