Donald Trump was in reach of a major political victory Thursday in the US Senate, where the president's controversial tax reform bill was expected to come to a vote no later than Friday.

The Republican majority appeared likely to adopt the bill -- in what would be one of the first-year president's most significant legislative victories -- though a few defections were possible and the vote was expected to be close.

The legislative psychodrama was shaping up in a way reminiscent of the Republican effort during the summer to repeal the Obamacare program: in the middle of the night, three senators, including John McCain, torpedoed the repeal that Trump had repeatedly promised during his election campaign.

This time, however, the Republican majority appears more solidy united -- McCain himself said Thursday he was on board -- around the underlying philosophy of tax reform: a significant reduction of corporate tax rates and smaller cuts for most individuals, along with a simplification meant to allow Americans to file their taxes "on a postcard."

These changes, Republicans insist, will bring relief to a financially strained middle class while sharply boosting the nation's economic growth.

"I believe this legislation, though far from perfect, would enhance American competitiveness, boost the economy, and provide long overdue tax relief for middle-class families," McCain said in a statement.

But some doubt remained Thursday whether all 52 Republican senators in the 100-member chamber will ultimately support the plan.

Some conservatives bridle at seeing their party trample the anti-deficit budget orthodoxy they preached during the Obama presidency: the bill before them would balloon the national deficit by $1.4 trillion over the coming decade, the Congressional Budget Office estimates.

Other senators have raised other, narrower concerns: over the tax rates for certain smaller companies; deductions for families with children; or a provision, slipped into the legislation, to end the requirement that all taxpayers carry health insurance or pay a fine -- a central pillar of the Obamacare program.

And the corporate tax rate might end up being 21 or 22 percent, rather than the 20 percent Trump has called for (the current rate is 35 percent).

"The Republicans have the Senate. The Republicans have the House. The Republicans have the White House. It's very unusual," Trump said in a speech Wednesday in the Midwestern state of Missouri. "This is our chance to free our economy and our workers from the terrible tax burden in Washington."

"We're going to keep winning and winning and winning and winning," he said.

- Biggest tax-law change in years -

If the legislation passes the Senate, it will still have to be reconciled with a House version passed November 16. Though Republicans control both chambers, their tax plans differ on some points.

As Republican leaders scrambled Thursday to overcome final objections and ensure the votes needed for passage, their essential argument was a simple one: If they fail to pass the tax bill they can expect major losses in next November's midterm elections.

Senator Lindsey Graham, a South Carolina Republican, summed up the challenge this way:

"The bottom line is, 52 people are trying to buy a car ... We may not all agree on the color or the type car, but if we don?t get a car, we're all going to walk. And most people don't want to walk. They'd rather have a car."

In Graham's folksy terms, senators will spend Thursday voting on a series of amendments over the color and make of their future car.

- Democrats see boost for the rich -

Democrats, united in opposition to the legislation, are arguing that the car is too expensive and will accommodate only the rich.

Analyses suggest that most people at all income levels will get immediate help from the tax cuts, but that the wealthiest five percent of households -- including many Republican donors -- will profit disproportionately.

And about half of all taxpayers will see their tax bills go up starting in 2027, according to the Tax Policy Center.

So, will Republicans stand together when the time comes to vote?

Senate Republican leader Mitch McConnell believes so.

"I'm optimistic," he told Fox News late Wednesday, that "we're going to deliver comprehensive tax reform to the American people for the first time in 31 years."