Obamacare repeal and tax reform will take center stage next week in Congress, where Republican lawmakers will be tested on whether they can finally fulfill the party's top campaign promises.

Prospects for Senate passage of a bill to partially repeal and replace Obamacare dimmed Friday when Sen. John McCain, R-Ariz., announced his opposition to the measure and instead called for working with Democrats on a bill to stabilize Obamacare.

Senate Republicans return Monday with just six days left to take up and pass a bill to repeal and replace Obamacare. If they don't vote before Sept. 30, a fiscal 2017 budget rule will expire and end their chance to avert a filibuster by Democrats.

The Senate bill, authored by Sens. Lindsey Graham, R-S.C., and Bill Cassidy, R-La., would turn Obamacare's federal financing into a block grant program allowing states to control health insurance delivery.

But passage with the required 51 votes now appears very difficult.

McCain said Friday the bill had not received a thorough examination through committee hearings and markups.

"I cannot in good conscience vote for the Graham-Cassidy proposal," McCain said in a statement. "I believe we could do better working together, Republicans and Democrats, and have not yet really tried. Nor could I support it without knowing how much it will cost, how it will effect insurance premiums, and how many people will be helped or hurt by it. "

Republican leaders must now turn to Sens. Lisa Murkowski of Alaska, Susan Collins of Maine, and Rand Paul of Kentucky, to try to round up the minimum 50 votes needed to pass Graham-Cassidy with Vice President Mike Pence serving as the tie-breaker.

But Paul has promised to vote against the bill because it does not fully repeal Obamacare. Collins is leaning against it because, she said, she believes it will hurt those with pre-existing medical conditions.

On Monday, the Senate Finance Committee will hold a hearing on the Graham-Cassidy measure. A preliminary analysis of the bill is also expected next week by the Congressional Budget Office.

Without McCain, however, it seems unlikely the bill can pass.

The House GOP in the meantime will begin moving in earnest on tax reform.

Republican leaders promised to present an outline of their tax plan this week and intend to hold hearings on legislation beginning in October.

On Wednesday, House GOP lawmakers will attend a half-day retreat specifically to discuss tax reform legislation and the need to include it in a 2018 budget resolution in order to circumvent a filibuster by Democrats in the Senate.

Republicans have been unable to agree on what to include or cut from a fiscal 2018 budget and have put off voting on a measure.

The House and Senate also plan to vote on an approximately six-month extension of the Federal Aviation Administration ahead of a Sept. 30 deadline.

Congress failed to pass a long-term comprehensive reform of the FAA this year due partly to disagreements over whether to remove air traffic control from the FAA and place it under the authority of a non-profit board.

In addition to the FAA bill, the House will vote to extend funding for a federal program providing home visits for low-income new mothers and babies.

The House will also vote on legislation that would suspend federal benefits for those who are subjects of outstanding arrest warrants for a felony or for violating parole or probation.