Another 2.8 million people could be added to the unemployment rolls at midnight if Congress fails to approve a bill to keep the government running, or if President Reagan vetoes the one Congress sends him.

Most everyone hopes this game of high-stakes, bluff poker (with real people and programs serving as the chips) will be resolved so Congress can take its Christmas break.

But with Uncle Sam facing his third shutdown of the year because most agency budgets have still not been approved, here are some "if" items to ponder:

* If the government should briefly go out of business most federal employes will not be paid for time on furlough. No work means no pay. Except for Congress, of course!

This latest money crunch results from congressional failure to approve budgets for many agencies for the fiscal year that started last October. Some agencies have been operating without budgets -- on so-called continuing resolutions -- for more than two years.

Congress has managed to clear some budgets, including the one for Congress. This means that even if most of the government is forced to temporarily fold its tent, our lawmakers will continue to get paid even if most other government workers don't, and if most other government services slow down or come to a stop.

* Come what may, the Army, Navy and Air Force will remain on the job. Provided of course the shutdown, if it happens, doesn't last too long. History teaches that it is dumb not to pay your army.

In an earlier shutdown this year the Defense Department kept going with some financial sleight of hand. The department withheld sending Social Security deductions (from military paychecks) to the Treasury and used the funds to meet ends. That sort of creative financing is possible only for a short period of time, and only with the president's blessing.

* If there is a shutdown, and if it lasts awhile, most federal workers would be eligible for unemployment compensation. Fewer than 30,000 ex-feds now collect unemployment benefits.

Ironically, it would be up to federal agencies to pay unemployment benefits -- via state programs -- to employes they had to let go because they didn't have the money to pay them to work.

Don't Do It! Reps. Steny Hoyer (D-Md.) and Frank Wolf (R-Va.) are trying to get support for a "sense of congress" resolution urging President Reagan to keep the government operating even if a continuing resolution isn't completed by the midnight deadline.

Hoyer and Wolf are members of the Post Office-Civil Service Committee. And they represent a lot of federal workers.