But most immediately, McCain may provide a critical vote in support of beginning formal debate on the health-care bill.
Trump's tweet is especially notable given that during the 2016 campaign, Trump questioned whether McCain should be considered a war hero because he was "captured". "If for some reason we aren't able to muster the votes tomorrow, which I'm increasingly optimistic we will, it's not the end of it", he said. "And you know, I made an extremely good showing against Senator McCain against all odds".
Yet fellow Republicans took McCain's return as a positive sign for Tuesday's vote.
Senate Republicans are so pessimistic about their looming Tuesday vote to move to repeal Obamacare that some of them now believe that falling a vote or two short would count as a win.
But no one, including the senators expected to vote on Tuesday, is sure what McConnell is planning.
With McCain's return - and barring any other absences - there will now be a full compliment of senators on hand for the health care test vote.
McCain was absent last week after having surgery to remove a blood clot from above his left eye, which then revealed his cancer.
UN Security Council to hold urgent meeting on Jerusalem violence
On Friday, three protesters were killed by Israeli troops, including a 17-year-old who was fatally shot in the neck. On Friday, three Israeli settlers were stabbed to death at a settlement near Ramallah in the occupied West Bank.
Paul tweeted that McConnell told him the plan was to take up the bill that repealed Obamacare that House and Senate Republicans passed in 2015 but was vetoed by President Barack Obama - even though it requires 60 votes and will be defeated. That would set up a House-Senate conference to resolve the differences between the two proposals, buying Republicans more time to continue to try to find a broad bill that repeals and replaces the ACA.
Beyond the motion to proceed vote and the ensuing debate, no one is quite sure - not even Republicans - what of substance the GOP will try to pass to overhaul health care, which affects one-sixth of the USA economy.
Trump would change his tune in a hurry if McCain voted no. During a radio interview, just the day after McCain's diagnosis was publicly announced, she suggested that he should step aside and clear the way. for her, probably.
'I'm dumbstruck by Kelli Ward's comments, ' Flake said.
Average survival for malignant glioblastoma, a condition that claimed the life of former Democratic Senate lion and McCain friend Ted Kennedy, tends to be around 14 months with treatment, that includes chemotherapy and radiation.
Efforts to resurrect McConnell's Obamacare replacement suffered a further blow on Friday when the Senate parliamentarian issued a preliminary finding that key parts of the proposal don't qualify for a fast-track procedure being used by the GOP.
One of the hosts said Ward is already "dancing" on McCain's grave, called her a "vulture" and told her she's "desperate for attention".