"My belief is, it's going to be a very big win, especially at the U.S. Senate level," Priebus said. "And we may add some seats in congressional races."
The party chairman, speaking to reporters at a Christian Science Monitor breakfast, said the year was turning out to be a political "disaster" for Democrats. He said the still-unpopular health care law and President Obama's sagging approval rating were creating several new Republican paths to victory.
Not so, Democratic National Committee Chairman Debbie Wasserman Schultz said in a rebuttal this morning.
Wasserman Schultz dismissed Priebus' bullishness about 2014, saying she's confident that the Senate will remain in Democratic hands, and that Republican predictions will turn out to be as accurate as their 2012 presidential bets.
"I'm confident that we're going to hold the Senate," Wasserman Schultz told reporters at the National Press Club today.
"They were predicting up to hours before the polls closed on Election Day in 2012 that we would be inaugurating President Romney too," she said. "So their prediction accuracy isn't always on the mark of late."
Why is the GOP so bullish, and is it warranted?
Voters in a midterm election are much more aligned with Republicans - older, whiter, etc. - which was most recently on display in the 2010 congressional elections, when Republicans swept control of the House. At the same time, Democrats have long had difficulty turning out their coalition of voters during nonpresidential years.
Priebus conceded that the party still faces significant "demographic challenges," particularly among young women, black and Hispanic voters. In rebuilding for the 2016 presidential election, he declared, "We'll work like dogs to try to figure it out."
"Democrats have demographic challenges too," Priebus said. "But for whatever reason we seem to be obsessed with our own."
If Republicans win control of the Senate in November - they only need six seats - the expectations among the party's conservative base will be sky-high.
Republicans would have full control over Congress, which would set up a two-year standoff with the White House. Because of that, some Republicans are beginning to wonder whether the good fortunes of 2014 could complicate the party's larger goal of winning back the White House in 2016, particularly if those expectations aren't met and the divide inside the Republican Party deepens.
Priebus played down those concerns today. That's a challenge, he said, to worry about after the midterm elections.
"If you have a Republican Senate," he said, "you have a much greater opportunity to box the president in."
As far as 2016 is concerned, being relatable is key.
"Put a candidate on the ballot that people want to sit down and have a beer with," Priebus said.
ABC News' Shushannah Walshe and Abby D. Phillip contributed to this report.