With 88% of precincts reporting in the northern industrial state of Michigan, Kerry had a commanding 50% of the vote. Former Vermont governor Howard Dean was next with 17% followed by North Carolina Senator John Edwards with 14%.
In the north-western state of Washington, Kerry was on top with 48.5% of 76% of precincts reporting. Dean followed with 30.5%, Edwards 6.4% and retired NATO general Wesley Clark with 3.2%.
With the two latest victories, Kerry - the 60-year-old Vietnam War hero - has won in nine of the 11 primaries held so far.
The winner of the Democratic nomination will challenge President George Bush in the presidential elections scheduled for November.
Kerry had been heavily favoured to win in Michigan and Washington and further tighten his grip on the nomination.
Kerry is also ahead in opinion polls in the north-eastern state of Maine, which will hold its caucus on Sunday.
Two southern states - Tennessee and Virginia - hold primaries on Tuesday.
Saturday brought more good news for Kerry as an opinion poll of registered voters conducted by Newsweek magazine found that he would beat President Bush by 50% to 45% if the presidential election were held now.
He also received the endorsement of former senator George Mitchell. "I support John Kerry because I believe that he is best able to advocate and advance the principles of our party and to win in November," Mitchell said.
Kerry has raced to the front of the pack largely due to the perception that he is the best equipped to take on Bush.