Republicans should expect Democrats to make up ground against the GOP in the four weeks between now and the election, House Minority Whip Eric CantorEric Ivan CantorRace for Republican Speaker rare chance to unify party for election Scalise allies upset over Ryan blindside on McCarthy endorsement 2018 will test the power of political nobodies MORE (R-Va.) said Monday.

Cantor, the second-ranking House Republican, who would likely become majority leader under a GOP-controlled House, sought to warn his party's rank-and-file members against becoming complacent about impending victories in pivotal midterm elections.

"I think that it's been predicted at this point that the poll numbers will tighten up," Cantor said during an appearance on CNBC.

Cantor said President Obama's poor approval ratings weighed the most on Democratic candidates.

Some recent polling suggests Democrats might be making up ground against Republicans, who have held a steady advantage over the majority party in recent polling. Democrats seized on a Newsweek poll showing a 5-point advantage for them in the generic congressional ballot, though that poll seems more like an aberration, given the slew of other polls suggesting a GOP advantage, or, at the very least, a tie between the two parties. Democrats also promoted their best fundraising month of the cycle in September as a sign of increased momentum in their direction.

Cantor suggested any tightening up in the race for control of Congress might be due to Obama and Democrats' increased focus on energizing their own base, which they hope will come to the polls Nov. 2 in full force.

"I do think that they're pivoting now to send a signal to the Democratic, left-wing base," Cantor said, pointing to the increased money that's been spent in competitive House and Senate races.

The second-ranking House Republican is also trying to set expectations for GOP voters, whom Republicans can't afford to have stay home if they want to pick up the 39 seats necessary to win control of the House.

"Everybody says that this is the 'silly season,' when we've got four weeks left and the money that's coming into a lot of the races, both of the House and the Senate, is unprecedented," he said.

Cantor also claimed a 13 percent advantage for the GOP among likely voters. Though he said this would also narrow, Cantor described it as an unprecedented advantage for the GOP in the last month of the campaign.