Story highlights Tuesday's vote allows debate to begin before a final vote expected later this week

Sens. Thad Cochran, Richard Shelby and Bob Menendez missed the vote

(CNN) The GOP-controlled Senate voted 50-47 on Tuesday to begin debate on the 2018 budget resolution, a procedural step toward laying the groundwork for passing tax reform.

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.

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