In a sign that the list of competitive states in the presidential race is expanding, Hillary Clinton and Donald Trump are locked in tight contests in four states, including two that haven't been battlegrounds for decades, new Wall Street Journal/NBC News/Marist polls find.

The polls could be a warning sign for Mrs. Clinton, the Democratic nominee, as they found the races in New Hampshire and Nevada—two states that Democrats have won in recent presidential elections—to be neck-and-neck.

Mrs. Clinton, in head-to-head matchups against Mr. Trump, leads by a single percentage point among likely voters in both states, 42% to 41% in New Hampshire and 45% to 44% in Nevada.

But the polls also spotted trouble for Mr. Trump, the Republican nominee, in unexpected places. The races are tight in two states that Republicans can usually take for granted: Arizona, where Mr. Trump leads by just 1 percentage point, 42% to 41%, and Georgia, where his lead is 46% to 43%.

The competition between the two remains equally close when the field is widened to include Libertarian nominee Gary Johnson and Green Party candidate Jill Stein.