WASHINGTON — Senate Republican leaders, trying to keep alive their flagging effort to dismantle the Affordable Care Act, are barreling toward a showdown vote on Tuesday to begin debating a repeal of the health law. But senators have yet to be told precisely what legislation they will be debating.

Apparently short of votes even to begin that process, President Trump ratcheted up pressure on Monday for Republican senators to get onboard, criticizing their inaction and warning that they risked betraying seven years of promises to raze and revamp the health law if they did not.

“Remember ‘repeal and replace,’ ‘repeal and replace’ — they kept saying it over and over again,” Mr. Trump said at the White House, flanked by people who he said suffered as “victims” of the “horrible disaster known as Obamacare.”

“Every Republican running for office promised immediate relief from this disastrous law,” the president said. “But so far, Senate Republicans have not done their job in ending the Obamacare nightmare.”

Tom Perez, the Democratic National Committee chairman, countered, “No matter how many ways President Trump tries to twist or hide the truth, the facts won’t change: The Affordable Care Act has been a lifesaver for millions of Americans.”

The remarks from Mr. Trump, who has been largely absent from the policy debate, had the ring of a threat by a president who has grown frustrated watching Republicans repeatedly try, and fail, to reach consensus on his campaign promise to immediately roll back the health law and enact a better system.

He said their constituents would exact a price for inaction — “you’ll see that at the voter booth, believe me” — and hinted that any Republican who did not support the bid to open debate on an as-yet-determined health bill would be painted as complicit in preserving a health law passed on the basis of “a big, fat, ugly lie.”

“For Senate Republicans, this is their chance to keep their promise,” Mr. Trump said, repeating the “repeal-and-replace” mantra on which Republicans campaigned last fall. “There’s been enough talk and no action; now is the time for action.”

After months of planning, debating and legislating, much of it behind closed doors, the Senate this week has reached the moment when votes will have to be cast. The fight on the Senate floor will unfold in stages.

First, the Senate majority leader, Mitch McConnell of Kentucky, said he would move ahead with a procedural vote on Tuesday to take up the health bill that narrowly passed the House in May. He urged his colleagues to do so.

“Many of us have waited literally years for this moment to finally arrive, and at long last, it has,” Mr. McConnell said on the Senate floor.

If that vote succeeds, the Senate would then be able to consider numerous amendments, including complete substitutes for the House bill. But it remains unclear what would take its place, and Senate Republican leaders have…