"So great that John McCain is coming back to vote. Brave - American hero! Thank you John," Trump tweeted.
McCain, recently diagnosed with an aggressive form of brain cancer, will return to the Senate today to cast a critical vote on the Republican health care bill.
The 80-year-old Senator, who underwent surgery to remove a blood clot from above his left eye early this month, was diagnosed with a brain tumour known as a glioblastoma.
The Senator, who Chairs the powerful Senate Armed Services Committee, has also been very critical of Trump's policies in particularly those on the foreign policy and national security front.
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McCain's office last night announced that he would return to the United States Senate today -- a surprise to most in Washington who expected him to miss the crucial vote and return to Washington at a later date.
McCain is expected to get GOP leadership one vote closer to beginning debate on health care legislation, which is on the verge of collapsing.
McCain's presence on the Senate floor is seen as key to the possible passage of the health care bill, which is a top priority for Trump.
With just 52 seats, the ruling Republican party has a wafer-thin majority in the 100 member Senate.
Trump in another tweet said that this would be another interesting day for healthcare.
"This will be a very interesting day for HealthCare. The Dems are obstructionists but the Republicans can have a great victory for the people!" he said.