Rep. Tom CottonTom CottonOvernight Cybersecurity: White House adviser ditches cyber panel over 'fake news' | Trump cyber order 'close' | GOP senator pushes for clean renewal of foreign intel law Overnight Tech: Dem wants to see FCC chief's net neutrality plans | New agency panel on telecom diversity | Trump calls NASA astronaut GOP senator pushes for clean reauthorization of foreign intel law MORE (R-Ark.) has edged Sen. Mark PryorMark PryorMedicaid rollback looms for GOP senators in 2020 Cotton pitches anti-Democrat message to SC delegation Ex-Sen. Kay Hagan joins lobby firm MORE (D-Ark.) in his fourth-quarter fundraising totals, bringing in $1.2 million to Pryor's $1.1 million.

Pryor has more money in the bank, with $4.2 million to Cotton's $2.2 million. But it's rare that challengers outraise incumbents this early in the campaign — and it's the latest signal of how tough a fight Pryor is in to retain his seat.

While both hauled in big sums, neither candidate did much to increase their money in the bank, a sign of how expensive the race has already become. Cotton added approximately $300,000 to his warchest, while Pryor's total actually dipped by about $200,000 in that period. Both candidates have already been on television with ads.

Pryor is widely viewed as the most vulnerable incumbent senator facing reelection this year. Polls have found a tight race in the state.

"Arkansans are supporting Tom Cotton because he shares their values... Arkansas doesn't need a senator who will hand his voting card to Harry Reid whenever he asks for it," Cotton Campaign Manager Justin Brasell said in a statement announcing the numbers. 

"Heading into the election year, Mark will have every resource to remind voters about his record as a reliable and responsible voice for Arkansas families, while drawing clear contrasts with Congressman Cotton's reckless agenda that puts his own interests ahead of Arkansans," Pryor spokesman Erik Dorey fired back.