Hillary Clinton is leading Donald J. Trump in Colorado, Virginia, North Carolina and Florida, according to a new set of polls that show the presumptive Democratic presidential nominee with solid advantages in four crucial swing states.

According to NBC/Wall Street Journal/Marist surveys, Mrs. Clinton leads Mr. Trump by a margin of 44 percent to 37 percent in Florida; 43 percent to 35 percent in Colorado; 44 percent to 35 percent in Virginia; and 44 percent to 38 percent in North Carolina.

The states account for 66 electoral votes and give Mrs. Clinton a strong foothold as the Republicans and Democrats head to their conventions in Cleveland and Philadelphia this month.

The polls show that large numbers of voters in each state remain undecided between the two leading candidates, and that both are viewed unfavorably by the electorate.