Pryor's re-election campaign on Thursday called Cotton's vote against the student loan measure a sign that the Republican is "far outside the mainstream." Cotton was one of six Republicans in the House who voted against the measure on Wednesday.
The bill links student loan interest rates to the financial markets. The bill would offer lower rates for most students now but higher rates down the line if the economy improves as expected.
Cotton, who is expected to join the Senate race next week, defended his vote against the measure.