Last Updated Jun 26, 2017 11:23 AM EDT

The Senate is days away from voting on a Republican plan to repeal and replace Obamacare, but leadership has, for the time being, fallen short so far of securing the 50 votes necessary to pass the measure.

Republicans actually need 51 votes to pass the legislation, but one of them can be Vice President Mike Pence's tie-breaking vote. Taking that into consideration, and assuming all Democrats vote against it, Republicans can only afford two defections. The Senate currently has 52 Republicans and 48 Democrats.

The 142-page measure, released Thursday, would end Obamacare's penalties for people who don't buy insurance, cut back an expansion of Medicaid, but would keep protections for people with pre-existing conditions, compared to the House-passed bill. It would provide tax credits, based on income, age and geography, which would make more money available to lower-income recipients to help them buy insurance. This differs from the House bill, which tied its tax credits only to age. Obamacare taxes would be repealed under the bill. The Senate bill would provide for expanded tax-free Health Savings Accounts, and it would also eliminate federal funding for Planned Parenthood for one year.

Hours after text was revealed Thursday, Sen. Rand Paul, R-Kentucky, announced with three other Senate Republicans that they opposed the bill in its current form because it doesn't go far enough in repealing President Obama's health care law. A day later, Sen. Dean Heller, R-Nevada, became the fifth Senate Republican to reject the measure.

However, unlike the hardliners opposiing the bill, Heller said he can't support the current version of the legislation because he believes it goes too far in cutting the expansion of Medicaid and would likely would take away health care coverage from millions of Americans and tens of thousands of Nevadans.

Some moderates, including Sen. Susan Collins, R-Maine, have also expressed concern.

Senate Majority Leader Mitch McConnell, R-Kentucky, has a challenging road ahead as he faces trying to bridge the divide between conservatives who want a more robust repeal of Obamacare and moderates who are wary about phasing out Obamacare's Medicaid expansion.

Here's a list of Senate Republicans who oppose the bill in its current form:

If three of the five continue to oppose the bill, and no Democrats support the bill, the measure will not pass. And even if McConnell is successful in getting it through the Senate, the process won't be over. The House and Senate would then have to reconcile their two bills and pass a final agreement in each chamber before it can be sent to President Trump's desk.

A cost estimate of the bill from the nonpartisan Congressional Budget Office (CBO) is expected to come out as early as Monday. The CBO's score of the House measure, which passed the lower chamber on May 4, found 23 million more people would be without health insurance over the next decade under the bill.

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Senate GOP health care bill vote: The whip count - CBS News