If the Senate passes the budget later this week, as is broadly expected, it would allow the Senate to pass a Republican leadership-backed tax reform plan with 51 votes instead of the 60 often needed for major legislation.
On Tuesday's procedural vote, Republican Sens. John McCain of Arizona and Rand Paul of Kentucky voted yes. McCain in a statement committed to supporting the underlying budget resolution, while Paul has yet to do so. They clashed with each other over whether to boost defense spending.
Their votes were needed because Republicans have a small 52-48 seat advantage. Two Republicans were missing: Sen. Thad Cochran of Mississippi was out sick
and Sen. Richard Shelby of Alabama was gone to attend a funeral, according to a GOP leadership aide. Cochran's office announced that the Mississippi senator had returned to Washington Tuesday.
Paul is unhappy that McCain and Sen. Lindsey Graham, R-South Carolina, want to increase the amount of defense funding allowed by the budget, something Paul said would break existing budget caps.
"Senators McCain and Graham are torpedoing the budget by insisting on busting the budget caps for more spending," Paul tweeted Tuesday. "I have told the White House and GOP leaders that if they simply stick to their own caps, the rest of the Budget is fine and I'll vote yes."
But McCain, who chairs the armed services committee, is holding firm. He has argued for years that defense funding has been kept artificially low because of budget caps.
"We are working on an arrangement where we can hopefully increase defense spending. I will not vote for anything that votes to cut defense spending," he said. "We have now more young men and women dying because of lack of readiness, maintenance and training that we are in combat. That's not acceptable."
Graham weighed in on Twitter, accusing Paul of peddling "bad info" about what McCain and Graham want.
"There's nothing in this budget that will lift spending caps," said Graham, who argued he and McCain would push to do that later during the appropriations process.
He warned Paul not to "screw up" by blocking tax reform as Graham thinks Paul did when he helped block the most recent GOP attempt to repeal and replace the Affordable Care Act.
GOP leaders, who can only afford to lose two Republican votes and still pass the budget, seem inclined to side with McCain in this argument.
Sen. John Thune of South Dakota, the third-ranking Republican leader, indicated that changes would not likely be made to appease Paul.
"Rand is going to make his views known. We'll see when push comes to shove where everybody is," he said. "But if you made changes in the budget to satisfy him you'd probably lose a number of other members. So it's always kind of a, you know, it's a balance you have to strike."
Democratic Sen. Bob Menendez also was absent because he is in New Jersey, where he is on trial for corruption charges
The Senate now begins up to 50 hours of debate before senators begin offering what can be an unlimited number of amendments in what is known as a vote-a-rama. Final passage of the bill is anticipated late Thursday night or early Friday morning.
This week's vote is not for the tax plan itself -- Republican leaders are still negotiating specifics of their legislation, though both GOP members of Congress and the White House have released frameworks
of what they want in a tax overhaul. House Speaker Paul Ryan has said he wants to pass tax legislation before the end of the year.