Republican Sen. Susan Collins is caught in the cross-fire of the angry partisan division that’s defining the nation’s politics, turning her from a shoo-in for re-election into one of the most vulnerable senators running in 2020. After spending four terms building a bipartisan brand in her home state of Maine, Collins has stoked anger from Democrats for her votes in favor of President Donald Trump’s tax cuts and his Supreme Court pick of Brett Kavanaugh, and from Republicans for helping save Obamacare and her frequent criticism of the president’s statements and behavior. Collins’ once sky-high approval ratings have plunged more than any other senator in the Trump era, and last week the non-partisan Cook Political Report put her Senate seat — one of just two in GOP hands in states won by Hillary Clinton and being contested in 2020 — in the toss-up category. The question for Collins, 66, is whether the power of incumbency and her reputation for bipartisanship can overcome a fierce Trump-era backlash. “The people of Maine have known me, and they know that I have been a hardworking, independent advocate for them, who votes with integrity,” she said in a series of interviews.