(Adds McCain expected to return to vote)

By Ayesha Rascoe and Amanda Becker

WASHINGTON, July 24 (Reuters) - President Donald Trump made a last-ditch plea to U.S. Senate Republicans on Monday to "do the right thing" and fulfill seven years of campaign promises to repeal and replace former President Barack Obama's signature healthcare law.

The Senate will vote on Tuesday on whether to open debate on an overhaul of the law, with Senate Majority Leader Mitch McConnell promising an open amendment process and a "robust" debate.

"To every member of the Senate I say this: The American people have waited long enough. There's been enough talk, and no action. Now is the time for action," Trump said on Monday at the White House.

Standing in front of families who he said had been hurt by the law popularly known as Obamacare, Trump said, "So far, Senate Republicans have not done their job in ending the Obamacare nightmare."

Even as it remained unclear on Monday whether McConnell had enough votes in the Senate to open debate, he said the vote would take place regardless.

"I know many of us have waited years for this moment to finally arrive. And, at long last, it finally has. I would urge every colleague to join me," McConnell said.

Senator John McCain, who has been battling brain cancer in his home state of Arizona, is expected to return to the Senate to cast a vote, his office said on Monday.

Moderate Senator Susan Collins, who has vocally opposed McConnell's efforts so far, said on Monday she would vote "no" on a motion to proceed.

Republicans have been under heavy political pressure to make good on their longstanding campaign promises to gut the 2010 law, which they view as a government intrusion in the healthcare market.

But the party is deeply divided between moderates concerned the Senate bill would eliminate insurance for millions of low-income Americans and conservatives who want to see even deeper cuts to the Obamacare legislation.

Senate Republicans have been unable to reach consensus on an approach, with McConnell failing to secure enough votes for either a repeal and replacement of Obamacare or a straight repeal.

Republicans hold a 52-48 majority in the 100-member Senate. With Democrats united in opposition, McConnell can only afford to lose two Republican votes.

'FORGOTTEN VICTIMS'

"The question for every senator, Democrat or Republican, is whether they will side with Obamacare’s architects, which have been so destructive to our country, or its forgotten victims?" Trump said.

While Trump has repeatedly called on Republicans to repeal and replace Obamacare, he has shown little interest in the policy specifics. Trump last week initially suggested he was fine with letting Obamacare collapse, then urged Republican senators to hash out a deal.