Very close Senate races 'in states Trump won big' is 'sea change': Dem Senate campaign chair Dems still have "a narrow path" to a Senate majority, Sen. Van Hollen said.

The Democrat leading his party's campaign efforts for the Senate said the fact that there are some "very close races" in states where President Donald Trump won overwhelmingly is a significant change.

"Let me just say that the fact that we still have a narrow path to a majority [in the Senate] is a sea change from where we were 18 months ago," U.S. Sen. Chris Van Hollen, head of the Democratic Senatorial Campaign Committee, told ABC News Chief Anchor George Stephanopoulos on "This Week" Sunday.

"These are some very close races and of course, they're in states Donald Trump won big, and it's a testament to these [Democratic] candidates that they're so focused on what matters to people in their state," the senator from Maryland said.

The Senate election map in 2018 favors Republicans because, of the 35 seats to be decided Tuesday, 26 are held by Democrats and nine by Republicans, meaning Democrats have to defend far more seats than the GOP. In addition, all but one of the seats Republicans are defending are in states won by Trump in 2016.

Van Hollen acknowledged that Senate Democrats face "the toughest political map in 60 years," but called it "remarkable" that the Democrats are "in as strong a position" as they are.

An example of Democrats' challenge in the Senate elections is in North Dakota, where Democratic Sen. Heidi Heitkamp is in a tight race for re-election against GOP challenger Rep. Kevin Cramer. FiveThirtyEight gives Heitcamp a one in four chance of winning.

But Van Hollen said Heitkamp shouldn't be counted out.

"In her last election six years ago, she was 10 points down with a week to go," Van Hollen said. "It's much closer than that in North Dakota. Don't count her out."

Democrats are also in a competitive race in New Jersey, where incumbent Democratic Sen. Bob Menendez is running against challenger Republican Bob Hugin. Menendez faces potential doubts from some voters after a federal corruption case against him last year ended in mistrial. ABC News rates the seat as likely to stay Democratic.

Van Hollen said he's "absolutely confident" that Menendez will win reelection in New Jersey.

"The people of New Jersey have been very clear," he said. "They do not want a rubber stamp for Donald Trump, and that is what Bob Menendez's opponent has done and will do."

Van Hollen said his party is seeing positive signs with high turnout in early voting so far, and he's confident that issues such as health care will drive Democrats to the polls on Tuesday.

FiveThirtyEight forecasts that Democrats have a 1 in 7 chance of winning the Senate, roughly the same odds that the Republicans have of maintaining the House, according to FiveThirtyEight's prediction.