By Nolan D. McCaskill

WASHINGTON (Reuters) -Maine Governor Janet Mills launched a U.S. Senate campaign on Tuesday, joining a crowded field of Democrats hoping to unseat Republican Senator Susan Collins with a campaign that intends to portray the incumbent as an out-of-step defender of President Donald Trump.

Mills, a 77-year-old popular two-term governor, enters the race as the establishment favorite. But her candidacy sets up a high-profile primary clash with oysterman Graham Platner, a progressive outsider who raised $3.25 million in the first six weeks of his campaign.

Mills, who is more moderate than Platner, is the only Democratic candidate in the race who has won elections.

Senator Catherine Cortez Masto, a Nevada Democrat, quickly endorsed Mills on Tuesday, calling her “a proven

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