Sen. Bernie Sanders (I., Vt.) said he did not believe there would be a "great blue wave" on Election Day, apparently trying to tamp down expectations for the Democratic Party.

"I happen not to believe that there’s going to be this great blue wave," he said at an event Sunday in Iowa. "I happen to believe that on election night, which party controls the U.S. House will come down to a very few seats."

He told the New York Times in an interview later that he was hopeful of a big Democratic victory but warned against overconfidence.

"I think that may happen," he said. "I’m doing everything I can to make it happen. But one thing I will absolutely guarantee you: It will not happen if people are sitting back and are cocky and talking about how sure they are of winning."

"I’m just issuing a warning, and that warning is that overconfidence will result in disaster," he added.

Democrats are bullish about winning back the House—Minority Leader Nancy Pelosi (D., Calif.) said Monday if the election were held today they would take the majority but face a difficult map to win a Senate majority.

Republicans control the White House and both houses of Congress, and the midterm of a president's term historically goes poorly for his party. President Barack Obama's Democrats took heavy losses in 2010 and 2014, as did George W. Bush's Republicans in 2006.

President Donald Trump's approval rating has ticked upward in recent weeks, although his popularity level tracks with facing a net loss of seats in November.

Sanders, a democratic socialist who caucuses with the Democrats, is one of potentially dozens of contenders for the 2020 Democratic nomination. His surprisingly strong run in 2016 against Hillary Clinton made him a left-wing icon, and he's in the middle of a nine-state campaign tour ahead of the midterms, hitting such states as Iowa, Indiana, Michigan, South Carolina, Wisconsin, Arizona, Colorado, Nevada and California.

He is not alone in cautioning against complacency. Democratic National Committee chair Tom Perez says he doesn't even use the term "blue wave."

"We always knew that this election was going to be close — I don’t use the term ‘blue wave,’ I always talk about the need for the blocking and tackling," Perez told CNN. "I always talk about the need for organizing, to make sure you’re leading with your values, and that’s how we’ve been winning throughout this year and throughout 2017."