WASHINGTON (Reuters) - The U.S. Senate prepared Wednesday for a final marathon session on healthcare reform, with Republicans forcing Democrats into a series of politically tough votes before senators can pass the last changes to the landmark law.

The day after President Barack Obama signed the dramatic overhaul of the $2.5 trillion healthcare system, the Senate hustled to finish work on a separate companion package of changes sought by Obama and Democrats in the House of Representatives.

Republicans offered at least two dozen amendments designed to derail the package or force Democrats to take a tough political stance ahead of November's congressional elections.

The amendments included proposals to deny erectile dysfunction drugs to sex offenders, to preserve savings from Medicare for the federal health program for the elderly and to prevent tax hikes for families earning less than $250,000.

The Senate's adoption of even one amendment would send the whole package back to the House for approval, just days after the House passed the sweeping $940 billion overhaul in a close vote that capped a year-long political struggle on healthcare.

Senate Democrats promised to reject the Republican amendments and put the finishing touches on the healthcare debate in a marathon round of voting expected to begin Wednesday night.

"We've come a long way," Senate Republican leader Harry Reid told reporters, ridiculing the Republican attempt to force Democrats into politically embarrassing stances.

"How serious can they be?" Reid said of Republicans. "Offering an amendment dealing with Viagra for rapists?"

Republicans promised to keep fighting the overhaul, saying the amendments were part of their effort to shine a light on a power-grab by Democrats in Washington.

'SEE WHAT'S BEING DONE'

"We're going to bring these issues up today and tomorrow, as long as we can keep this debate going so Americans can see what's being done here," Republican Senator Jim DeMint said on MSNBC.

The package of changes to the healthcare overhaul include an expansion of subsidies to make

insurance more affordable and more state aid for the Medicaid program for the poor.

It also would eliminate a controversial Senate deal exempting Nebraska from paying for Medicaid expansion costs, close a "doughnut hole" in prescription drug coverage and modify a January deal on a tax on high-cost insurance plans.

The final package would extend taxes for Medicare, the federal health insurance program for the elderly and disabled, to unearned income. It also includes an overhaul of the student loan program.

The sweeping overhaul signed by Obama represents the biggest changes to the healthcare system in four decades. It expands insurance coverage to 32 million Americans and imposes new insurance regulations like barring companies from refusing to cover patients with pre-existing medical conditions.

The health insurance industry has been critical of the plan, but the Morgan Stanley Healthcare Payor index of health insurers has risen about 1 percent since passage as investors were encouraged that clarity on health reform was near and the bill avoided worst-case scenarios.

Shares of the largest two health insurers, UnitedHealth Group and WellPoint Inc, slumped more than 3 percent and 2 percent respectively in the same period, giving up some of their gains from last week.

The Senate is considering the "fixes" under budget reconciliation rules that allow approval on a simple majority, rather than the 60 votes needed to overcome Republican procedural hurdles.

Democrats control 59 votes in the Senate after losing their crucial 60th vote in January in a special Senate election in Massachusetts.

Obama signed an executive order reaffirming a ban on using federal funds to pay for abortions, part of a deal with about a half-dozen anti-abortion House Democrats that won their support Sunday.

The Democratic abortion rights opponents in the House had said they were concerned the restrictions on federal abortion funding were not strong enough in the healthcare bill.

(Additional reporting by Lewis Krauskopf, David Morgan; Editing by David Alexander and Cynthia Osterman)