Major celebrities are working behind the scenes to secure prominent appearances at the upcoming Democratic National Convention ... but the Kamala Harris campaign isn't accommodating everyone ... for fear of making the DNC feel too Hollywood.

Sources with direct knowledge tell TMZ … the Harris-Walz campaign wants to be smart about celebrity involvement, and is working hard to avoid the Chicago-based DNC being painted as a "Hollywood liberal elite" event.

Our sources say the campaign feels Hillary Clinton gave celebrities too big a platform back in 2016 and it backfired on her ... and they don't want to overdo the Hollywood aspect to the point where it turns off everyday folks.

In other words, it's NOT the more the merrier as far as celebrity appearances are concerned. Instead, our sources say the DNC and the Harris-Walz campaign are being more strategic and selective ... and picking their spots with celeb speakers and performers that make sense.

It'll be interesting to see who gets invited to the DNC when the dust settles .. our sources say plans are still being finalized and tons of big artists are vying for convention slots and their own campaign events with Harris and Walz.

When President Biden dropped out of the race and endorsed Harris, we're told her people were flooded with inquiries from A-list celebs and musicians looking to get involved in her campaign and the DNC.

But, with concerns about turning the DNC into a Hollywood production, it sounds like the Harris-Walz campaign is going to have to make some tough choices ... and turn a lot of celebs away.

We reached out to the Harris For President campaign, and senior spokesperson Adrienne Elrod tells TMZ ... "We are living in the most personalized media environment ever, so it's crucial we are leveraging the unparalleled excitement around this historic ticket by doing everything we can to break through to the voters who will decide this election."

Elrod adds ... "Celebrities bring new voters into the fold by speaking authentically to their fan bases about the election and the issues at stake in ways only they can."

We do know John Legend will be part of the program, though we don't know if he'll be speaking or performing.